diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -319,9 +319,6 @@
   assert(IncomingBlocks.size() == 2 &&
          "Only for a pair of incoming blocks at the time!");
 
-  // FIXME: it is okay if one of the incoming values is an `undef` value,
-  //        iff the other incoming value is guaranteed to be a non-poison value.
-  // FIXME: it is okay if one of the incoming values is a `poison` value.
   return all_of(BB->phis(), [IncomingBlocks, EquivalenceSet](PHINode &PN) {
     Value *IV0 = PN.getIncomingValueForBlock(IncomingBlocks[0]);
     Value *IV1 = PN.getIncomingValueForBlock(IncomingBlocks[1]);
@@ -330,6 +327,19 @@
     if (EquivalenceSet && EquivalenceSet->contains(IV0) &&
         EquivalenceSet->contains(IV1))
       return true;
+
+    // It is okay if one of the incoming values is an `undef` value, iff the
+    // other incoming value is guaranteed to be a non-poison value.
+    if (isa<UndefValue>(IV0) && isGuaranteedNotToBePoison(IV1))
+      return true;
+    if (isa<UndefValue>(IV1) && isGuaranteedNotToBePoison(IV0))
+      return true;
+
+    // It is okay if one of the incoming values is a `poison` value.
+    if (isa<PoisonValue>(IV0) && !isa<PoisonValue>(IV1))
+      return true;
+    if (isa<PoisonValue>(IV1) && !isa<PoisonValue>(IV0))
+      return true;
     return false;
   });
 }