[InstCombine] If return-inst in unreachable refers to an inst change it to poison (#65107)

Instructions in unreachable basic blocks are removed, but terminators
are not. In this case, even instructions that are only referenced by
a terminator, such as a return instruction, cannot be processed
properly.

This patch changes the operand of a return instruction in an
unreachable basic block to poison if it refers to the instruction,
allowing the instruction to be properly processed.

Author: ParkHanbum

llvm/lib/Transforms/Utils/Local.cpp

@@ -2779,6 +2779,11 @@ llvm::removeAllNonTerminatorAndEHPadInstructions(BasicBlock *BB) {
   Instruction *EndInst = BB->getTerminator(); // Last not to be deleted.
   // RemoveDIs: erasing debug-info must be done manually.
   EndInst->dropDbgValues();
+
+  if (isa<ReturnInst>(EndInst) && EndInst->getNumOperands() > 0 &&
+      isa<Instruction>(EndInst->getOperand(0)))
+    EndInst->setOperand(0, PoisonValue::get(EndInst->getOperand(0)->getType()));
+
   while (EndInst != &BB->front()) {
     // Delete the next to last instruction.
     Instruction *Inst = &*--EndInst->getIterator();

llvm/test/Transforms/InstCombine/phi-select-constant.ll

@@ -140,12 +140,11 @@ end:
 define i16 @sink_to_unreachable_crash(i1 %a)  {
 ; CHECK-LABEL: @sink_to_unreachable_crash(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[S:%.*]] = select i1 [[A:%.*]], i16 0, i16 5
 ; CHECK-NEXT:    br label [[INF_LOOP:%.*]]
 ; CHECK:       inf_loop:
 ; CHECK-NEXT:    br label [[INF_LOOP]]
 ; CHECK:       unreachable:
-; CHECK-NEXT:    ret i16 [[S]]
+; CHECK-NEXT:    ret i16 poison
 ;
 entry:
   %s = select i1 %a, i16 0, i16 5