[Local] Verify opcodes match for all insts passed to mergeFlags (NFC). (#141231)

The logic for tracking flags relies on all instructions having the same
opcode. Add an assert to check, as suggested in
#140406.

PR: #141231

Author: fhahn

llvm/include/llvm/Transforms/Utils/Local.h

@@ -565,6 +565,12 @@ struct OverflowTracking {
   bool HasNSW = true;
   bool IsDisjoint = true;
 
+#ifndef NDEBUG
+  /// Opcode of merged instructions. All instructions passed to mergeFlags must
+  /// have the same opcode.
+  std::optional<unsigned> Opcode;
+#endif
+
   // Note: At the moment, users are responsible to manage AllKnownNonNegative
   // and AllKnownNonZero manually. AllKnownNonNegative can be true in a case
   // where one of the operands is negative, but one the operators is not NSW.

llvm/lib/Transforms/Utils/Local.cpp

@@ -4364,6 +4364,13 @@ bool llvm::inferAttributesFromOthers(Function &F) {
 }
 
 void OverflowTracking::mergeFlags(Instruction &I) {
+#ifndef NDEBUG
+  if (Opcode)
+    assert(Opcode == I.getOpcode() &&
+           "can only use mergeFlags on instructions with matching opcodes");
+  else
+    Opcode = I.getOpcode();
+#endif
   if (isa<OverflowingBinaryOperator>(&I)) {
     HasNUW &= I.hasNoUnsignedWrap();
     HasNSW &= I.hasNoSignedWrap();