[DivRemPairs] make sure we have a valid CFG for hoisting division

This transform was added with e38b7e8
and as shown in:
https://llvm.org/PR51241
...it could crash without an extra check of the blocks.

There might be a more compact way to write this constraint,
but we can't just count the successors/predecessors without
affecting a test that includes a switch instruction.

llvm/lib/Transforms/Scalar/DivRemPairs.cpp

@@ -272,9 +272,10 @@ static bool optimizeDivRem(Function &F, const TargetTransformInfo &TTI,
 
       if (PredBB && IsSafeToHoist(RemInst, RemBB) &&
           IsSafeToHoist(DivInst, DivBB) &&
-          llvm::all_of(successors(PredBB), [&](BasicBlock *BB) {
-            return BB == DivBB || BB == RemBB;
-          })) {
+          all_of(successors(PredBB),
+                 [&](BasicBlock *BB) { return BB == DivBB || BB == RemBB; }) &&
+          all_of(predecessors(DivBB),
+                 [&](BasicBlock *BB) { return BB == RemBB || BB == PredBB; })) {
         DivDominates = true;
         DivInst->moveBefore(PredBB->getTerminator());
         Changed = true;

llvm/test/Transforms/DivRemPairs/X86/div-expanded-rem-pair.ll

@@ -529,3 +529,35 @@ return:                                           ; preds = %if.then, %if.end3
   %retval.0 = phi i64 [ %div, %if.end3 ], [ 0, %if.then ]
   ret i64 %retval.0
 }
+
+; Negative test (this would create invalid IR and crash).
+; The div block can't have predecessors other than the rem block
+; and the common single pred block (it is reachable from entry here).
+
+define i32 @PR51241(i1 %b1, i1 %b2, i32 %t0) {
+; CHECK-LABEL: @PR51241(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br i1 [[B1:%.*]], label [[DIVBB:%.*]], label [[PREDBB:%.*]]
+; CHECK:       predbb:
+; CHECK-NEXT:    br i1 [[B2:%.*]], label [[DIVBB]], label [[REMBB:%.*]]
+; CHECK:       rembb:
+; CHECK-NEXT:    [[REM2:%.*]] = srem i32 7, [[T0:%.*]]
+; CHECK-NEXT:    br label [[DIVBB]]
+; CHECK:       divbb:
+; CHECK-NEXT:    [[DIV:%.*]] = sdiv i32 7, [[T0]]
+; CHECK-NEXT:    ret i32 [[DIV]]
+;
+entry:
+  br i1 %b1, label %divbb, label %predbb
+
+predbb:
+  br i1 %b2, label %divbb, label %rembb
+
+rembb:
+  %rem2 = srem i32 7, %t0
+  br label %divbb
+
+divbb:
+  %div = sdiv i32 7, %t0
+  ret i32 %div
+}