Avoid underflow in f2reduce (triton-lang#4168)

When running
[convert_blocked1d_to_slice0](https://github.com/triton-lang/triton/blob/0ba5f0c3cd029d5c3d1f01b9bf29dac32c27345e/test/Conversion/tritongpu_to_llvm.mlir#L924)
Triton ends up computing a rank of a matrix with 0 columns during linear
layout lowering, which trips up f2reduce, and causes undefined behavior,
detectable through
[UBSAN](https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html).

Fix this by returning the rank (0) early in these cases, without calling
f2reduce.

<details><summary>Stack trace</summary>
<p>

```
third_party/triton/third_party/f2reduce/f2reduce.cpp:421:30: runtime error: shift exponent 18446744073709551615 is too large for 64-bit type 'unsigned long long'
    #0 0x556ee2fea3be in inplace_rref_small third_party/triton/third_party/f2reduce/f2reduce.cpp:421:30
    #1 0x556ee2fea3be in f2reduce::inplace_rref_strided(unsigned long*, unsigned long, unsigned long, unsigned long) third_party/triton/third_party/f2reduce/f2reduce.cpp:470:9
    #2 0x556ee2ea70da in getMatrixRank third_party/triton/lib/Tools/LinearLayout.cpp:125:3
    #3 0x556ee2ea70da in mlir::triton::LinearLayout::checkInvariants(bool) third_party/triton/lib/Tools/LinearLayout.cpp:299:7
    #4 0x556ee2ea656d in mlir::triton::LinearLayout::tryCreate(llvm::MapVector<mlir::StringAttr, std::__u::vector<std::__u::vector<int, std::__u::allocator<int>>, std::__u::allocator<std::__u::vector<int, std::__u::allocator<int>>>>, llvm::DenseMap<mlir::StringAttr, unsigned int, llvm::DenseMapInfo<mlir::StringAttr, void>, llvm::detail::DenseMapPair<mlir::StringAttr, unsigned int>>, llvm::SmallVector<std::__u::pair<mlir::StringAttr, std::__u::vector<std::__u::vector<int, std::__u::allocator<int>>, std::__u::allocator<std::__u::vector<int, std::__u::allocator<int>>>>>, 0u>>, llvm::ArrayRef<std::__u::pair<mlir::StringAttr, int>>, bool) third_party/triton/lib/Tools/LinearLayout.cpp:190:41
    #5 0x556ee2eb2150 in mlir::triton::LinearLayout::divideRight(mlir::triton::LinearLayout const&) third_party/triton/lib/Tools/LinearLayout.cpp:654:51
    #6 0x556ee2ee1c39 in mlir::cvtNeedsSharedMemory(mlir::RankedTensorType, mlir::RankedTensorType) third_party/triton/lib/Analysis/Utility.cpp:652:14
    #7 0x556ee2cf38fd in mlir::triton::getRepShapeForCvtLayout(mlir::triton::gpu::ConvertLayoutOp) third_party/triton/lib/Analysis/Allocation.cpp:66:8
    #8 0x556ee2cf3efa in mlir::triton::getScratchConfigForCvtLayout(mlir::triton::gpu::ConvertLayoutOp, unsigned int&, unsigned int&) third_party/triton/lib/Analysis/Allocation.cpp:95:19
    #9 0x556ee2cf6057 in mlir::triton::AllocationAnalysis::getScratchValueSize(mlir::Operation*) third_party/triton/lib/Analysis/Allocation.cpp:272:24
    #10 0x556ee2cf5499 in operator() third_party/triton/lib/Analysis/Allocation.cpp:343:7
    #11 0x556ee2cf5499 in void llvm::function_ref<void (mlir::Operation*)>::callback_fn<mlir::triton::AllocationAnalysis::getValuesAndSizes()::'lambda'(mlir::Operation*)>(long, mlir::Operation*) third_party/llvm/llvm-project/llvm/include/llvm/ADT/STLFunctionalExtras.h:45:12
    #12 0x556edeeee7a9 in operator() third_party/llvm/llvm-project/llvm/include/llvm/ADT/STLFunctionalExtras.h:68:12
    #13 0x556edeeee7a9 in void mlir::detail::walk<mlir::ForwardIterator>(mlir::Operation*, llvm::function_ref<void (mlir::Operation*)>, mlir::WalkOrder) third_party/llvm/llvm-project/mlir/include/mlir/IR/Visitors.h:174:5
    #14 0x556edeeee87c in void mlir::detail::walk<mlir::ForwardIterator>(mlir::Operation*, llvm::function_ref<void (mlir::Operation*)>, mlir::WalkOrder) third_party/llvm/llvm-project/mlir/include/mlir/IR/Visitors.h:182:9
    #15 0x556ee2cf49e7 in walk<(mlir::WalkOrder)0, mlir::ForwardIterator, (lambda at third_party/triton/lib/Analysis/Allocation.cpp:341:42), mlir::Operation *, void> third_party/llvm/llvm-project/mlir/include/mlir/IR/Visitors.h:313:10
    #16 0x556ee2cf49e7 in walk<(mlir::WalkOrder)0, mlir::ForwardIterator, (lambda at third_party/triton/lib/Analysis/Allocation.cpp:341:42), void> third_party/llvm/llvm-project/mlir/include/mlir/IR/Operation.h:794:12
    #17 0x556ee2cf49e7 in mlir::triton::AllocationAnalysis::getValuesAndSizes() third_party/triton/lib/Analysis/Allocation.cpp:341:16
    #18 0x556ee2cf4852 in run third_party/triton/lib/Analysis/Allocation.cpp:182:5
    #19 0x556ee2cf4852 in AllocationAnalysis third_party/triton/lib/Analysis/Allocation.cpp:169:5
    #20 0x556ee2cf4852 in mlir::Allocation::run(llvm::DenseMap<mlir::FunctionOpInterface, mlir::Allocation, llvm::DenseMapInfo<mlir::FunctionOpInterface, void>, llvm::detail::DenseMapPair<mlir::FunctionOpInterface, mlir::Allocation>>&) third_party/triton/lib/Analysis/Allocation.cpp:627:3
    #21 0x556ee1677402 in operator() third_party/triton/include/triton/Analysis/Allocation.h:227:26
    #22 0x556ee1677402 in void mlir::CallGraph<mlir::Allocation>::doWalk<(mlir::WalkOrder)0, (mlir::WalkOrder)1, mlir::ModuleAllocation::ModuleAllocation(mlir::ModuleOp)::'lambda'(mlir::CallOpInterface, mlir::FunctionOpInterface), mlir::ModuleAllocation::ModuleAllocation(mlir::ModuleOp)::'lambda'(mlir::FunctionOpInterface)>(mlir::FunctionOpInterface, llvm::DenseSet<mlir::FunctionOpInterface, llvm::DenseMapInfo<mlir::FunctionOpInterface, void>>&, mlir::ModuleAllocation::ModuleAllocation(mlir::ModuleOp)::'lambda'(mlir::CallOpInterface, mlir::FunctionOpInterface), mlir::ModuleAllocation::ModuleAllocation(mlir::ModuleOp)::'lambda'(mlir::FunctionOpInterface)) third_party/triton/include/triton/Analysis/Utility.h:350:7
    triton-lang#23 0x556ee16756b3 in walk<(mlir::WalkOrder)0, (mlir::WalkOrder)1, (lambda at third_party/triton/include/triton/Analysis/Allocation.h:222:9), (lambda at third_party/triton/include/triton/Analysis/Allocation.h:224:9)> third_party/triton/include/triton/Analysis/Utility.h:242:7
    triton-lang#24 0x556ee16756b3 in mlir::ModuleAllocation::ModuleAllocation(mlir::ModuleOp) third_party/triton/include/triton/Analysis/Allocation.h:220:5
    triton-lang#25 0x556ee2c2bf18 in (anonymous namespace)::AllocateSharedMemory::runOnOperation() third_party/triton/lib/Conversion/TritonGPUToLLVM/AllocateSharedMemory.cpp:26:22
...
UndefinedBehaviorSanitizer: invalid-shift-exponent third_party/triton/third_party/f2reduce/f2reduce.cpp:421:30 
```
</p>
</details>

lib/Tools/LinearLayout.cpp

@@ -121,6 +121,11 @@ getInjectiveMat(const LinearLayout &layout) {
 // outDim as columns.  In other words, finds the number of linearly-independent
 // bases for this output dimension.
 int getMatrixRank(std::unique_ptr<uint64_t[]> m, int numRows, int numCols) {
+  // f2reduce underflows if the number of cols is 0, return the rank early in
+  // this case.
+  if (numCols == 0) {
+    return 0;
+  }
   // stride is specified in number of 64-bit words per row, and we pack our
   // matrix so that there's only one uint64_t per row.
   assert(numCols <= 64);