[llvm] [InstCombine] If inst in unreachable refers to an inst change it to poison (#65107) (PR #78444)

[Nikita Popov via llvm-commits]

================
@@ -2770,15 +2770,32 @@ bool llvm::replaceAllDbgUsesWith(Instruction &From, Value &To,
   return false;
 }
 
+void llvm::handleUnreachableTerminator(Instruction *I) {
+  // RemoveDIs: erase debug-info on this instruction manually.
+  I->dropDbgValues();
+  unsigned Opcode = I->getOpcode();
+  if (Opcode >= Instruction::Ret && Opcode <= Instruction::Invoke &&
+      I->getNumOperands() > 0) {
+    for (unsigned i = 0; i < I->getNumOperands(); i++) {
----------------
nikic wrote:

```suggestion
    for (Use &U : I->operands())
```
and then `U.set()`.

https://github.com/llvm/llvm-project/pull/78444