[llvm] [UnifyLoopExits] Never generate phis of only `undef` values (PR #99924)

[Johannes Doerfert via llvm-commits]

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@@ -1909,22 +1909,29 @@ static void reconnectPhis(BasicBlock *Out, BasicBlock *GuardBlock,
     auto NewPhi =
         PHINode::Create(Phi->getType(), Incoming.size(),
                         Phi->getName() + ".moved", FirstGuardBlock->begin());
+    bool AllUndef = true;
     for (auto *In : Incoming) {
       Value *V = UndefValue::get(Phi->getType());
       if (In == Out) {
         V = NewPhi;
       } else if (Phi->getBasicBlockIndex(In) != -1) {
         V = Phi->removeIncomingValue(In, false);
+        AllUndef &= isa<UndefValue>(V);
       }
       NewPhi->addIncoming(V, In);
     }
     assert(NewPhi->getNumIncomingValues() == Incoming.size());
+    Value *NewV = NewPhi;
+    if (AllUndef) {
+      NewPhi->eraseFromParent();
+      NewV = UndefValue::get(Phi->getType());
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jdoerfert wrote:

As of right now, this emits undef by default. We likely want to change both UndefValue::get to poison. Should I include that and track undef/poison separately?

https://github.com/llvm/llvm-project/pull/99924