Automerge: [LoopFuse] Change placeholder from undef to poison (#131535)

Use `poison` instead of `undef` as a placeholder for phi entries of
unreachable predecessors.

Author: pedroclobo

llvm/lib/Transforms/Scalar/LoopFuse.cpp

@@ -1968,7 +1968,7 @@ struct LoopFuser {
           PHINode::Create(LCV->getType(), 2, LCPHI->getName() + ".afterFC0");
       L1HeaderPHI->insertBefore(L1HeaderIP);
       L1HeaderPHI->addIncoming(LCV, FC0.Latch);
-      L1HeaderPHI->addIncoming(UndefValue::get(LCV->getType()),
+      L1HeaderPHI->addIncoming(PoisonValue::get(LCV->getType()),
                                FC0.ExitingBlock);
 
       LCPHI->setIncomingValue(L1LatchBBIdx, L1HeaderPHI);