[RISCV] Only return DestSourcePair from isCopyInstrImpl for registers

ADDI often has a frameindex in operand 1, but consumers of this
interface, such as MachineSink, tend to call getReg() on the Destination
and Source operands, leading to the following crash when building
FreeBSD after this implementation was added in 8cf6778:

```
clang: llvm/include/llvm/CodeGen/MachineOperand.h:359: llvm::Register llvm::MachineOperand::getReg() const: Assertion `isReg() && "This is not a register operand!"' failed.
PLEASE submit a bug report to https://bugs.llvm.org/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
 #0 0x00007f4286f9b4d0 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) llvm/lib/Support/Unix/Signals.inc:563:0
 #1 0x00007f4286f9b587 PrintStackTraceSignalHandler(void*) llvm/lib/Support/Unix/Signals.inc:630:0
 rust-lang#2 0x00007f4286f9926b llvm::sys::RunSignalHandlers() llvm/lib/Support/Signals.cpp:71:0
 rust-lang#3 0x00007f4286f9ae52 SignalHandler(int) llvm/lib/Support/Unix/Signals.inc:405:0
 rust-lang#4 0x00007f428646ffd0 (/lib/x86_64-linux-gnu/libc.so.6+0x3efd0)
 rust-lang#5 0x00007f428646ff47 raise /build/glibc-2ORdQG/glibc-2.27/signal/../sysdeps/unix/sysv/linux/raise.c:51:0
 rust-lang#6 0x00007f42864718b1 abort /build/glibc-2ORdQG/glibc-2.27/stdlib/abort.c:81:0
 rust-lang#7 0x00007f428646142a __assert_fail_base /build/glibc-2ORdQG/glibc-2.27/assert/assert.c:89:0
 rust-lang#8 0x00007f42864614a2 (/lib/x86_64-linux-gnu/libc.so.6+0x304a2)
 rust-lang#9 0x00007f428d4078e2 llvm::MachineOperand::getReg() const llvm/include/llvm/CodeGen/MachineOperand.h:359:0
rust-lang#10 0x00007f428d8260e7 attemptDebugCopyProp(llvm::MachineInstr&, llvm::MachineInstr&) llvm/lib/CodeGen/MachineSink.cpp:862:0
rust-lang#11 0x00007f428d826442 performSink(llvm::MachineInstr&, llvm::MachineBasicBlock&, llvm::MachineInstrBundleIterator<llvm::MachineInstr, false>, llvm::SmallVectorImpl<llvm::MachineInstr*>&) llvm/lib/CodeGen/MachineSink.cpp:918:0
rust-lang#12 0x00007f428d826e27 (anonymous namespace)::MachineSinking::SinkInstruction(llvm::MachineInstr&, bool&, std::map<llvm::MachineBasicBlock*, llvm::SmallVector<llvm::MachineBasicBlock*, 4u>, std::less<llvm::MachineBasicBlock*>, std::allocator<std::pair<llvm::MachineBasicBlock* const, llvm::SmallVector<llvm::MachineBasicBlock*, 4u> > > >&) llvm/lib/CodeGen/MachineSink.cpp:1073:0
rust-lang#13 0x00007f428d824a2c (anonymous namespace)::MachineSinking::ProcessBlock(llvm::MachineBasicBlock&) llvm/lib/CodeGen/MachineSink.cpp:410:0
rust-lang#14 0x00007f428d824513 (anonymous namespace)::MachineSinking::runOnMachineFunction(llvm::MachineFunction&) llvm/lib/CodeGen/MachineSink.cpp:340:0
```

Thus, check that operand 1 is also a register in the condition.

Reviewed By: arichardson, luismarques

Differential Revision: https://reviews.llvm.org/D89090

llvm/lib/Target/RISCV/RISCVInstrInfo.cpp

@@ -538,7 +538,9 @@ RISCVInstrInfo::isCopyInstrImpl(const MachineInstr &MI) const {
   default:
     break;
   case RISCV::ADDI:
-    if (MI.getOperand(2).isImm() && MI.getOperand(2).getImm() == 0)
+    // Operand 1 can be a frameindex but callers expect registers
+    if (MI.getOperand(1).isReg() && MI.getOperand(2).isImm() &&
+        MI.getOperand(2).getImm() == 0)
       return DestSourcePair{MI.getOperand(0), MI.getOperand(1)};
     break;
   case RISCV::FSGNJ_D:

llvm/test/CodeGen/RISCV/copy-frameindex.mir

@@ -0,0 +1,61 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -march=riscv64 -debugify-and-strip-all-safe -run-pass machine-sink %s -o - 2>&1 | FileCheck %s
+
+--- |
+  define void @sink_addi_fi(i32 %0) !dbg !5 {
+  bb.0:
+    %1 = alloca i32, align 4
+    call void @llvm.dbg.value(metadata i32* %1, metadata !1, metadata !DIExpression()), !dbg !3
+    %2 = icmp eq i32 %0, 0
+    br i1 %2, label %bb.2, label %bb.1
+  bb.1:
+    store volatile i32 0, i32* %1, align 4
+    br label %bb.2
+  bb.2:
+    ret void
+  }
+
+  declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+  !llvm.dbg.cu = !{!0}
+
+  !0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !4)
+  !1 = !DILocalVariable(name: "var", scope: !5, file: !4, line: 2, type: !2)
+  !2 = !DIBasicType(name: "unsigned int", size: 32, encoding: DW_ATE_unsigned)
+  !3 = !DILocation(line: 0, scope: !5)
+  !4 = !DIFile(filename: "test.c", directory: "")
+  !5 = distinct !DISubprogram(name: "sink_addi_fi", scope: !4, file: !4, line: 1, type: !6, scopeLine: 1, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !0, retainedNodes: !8)
+  !6 = !DISubroutineType(types: !7)
+  !7 = !{null, !2}
+  !8 = !{}
+
+...
+---
+name: sink_addi_fi
+liveins:
+  - { reg: '$x10', virtual-reg: '%0' }
+stack:
+  - { id: 0, offset: 0, size: 4, alignment: 4 }
+body: |
+  ; CHECK-LABEL: name: sink_addi_fi
+  ; CHECK: bb.0:
+  ; CHECK:   successors: %bb.2(0x40000000), %bb.1(0x40000000)
+  ; CHECK:   [[COPY:%[0-9]+]]:gpr = COPY $x10
+  ; CHECK:   BEQ killed [[COPY]], $x0, %bb.2
+  ; CHECK: bb.1:
+  ; CHECK:   successors: %bb.2(0x80000000)
+  ; CHECK:   [[ADDI:%[0-9]+]]:gpr = ADDI %stack.0, 0
+  ; CHECK:   SW $x0, killed [[ADDI]], 0 :: (volatile store 4 into %stack.0)
+  ; CHECK: bb.2:
+  ; CHECK:   PseudoRET
+  bb.0:
+    liveins: $x10
+    %0:gpr = COPY $x10
+    DBG_VALUE %stack.0, $noreg, !1, !DIExpression(), debug-location !3
+    %1:gpr = ADDI %stack.0, 0
+    DBG_VALUE %1, $noreg, !1, !DIExpression(DW_OP_plus_uconst, 0, DW_OP_stack_value), debug-location !3
+    BEQ killed %0:gpr, $x0, %bb.2
+  bb.1:
+    SW $x0, killed %1:gpr, 0 :: (volatile store 4 into %stack.0, align 4)
+  bb.2:
+    PseudoRET