[llvm] [SimplifyCFG] Skip phi scans when folded block shares no predecessors (PR #225647)

[Max Brandtner via llvm-commits]

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@@ -963,6 +961,25 @@ static void gatherIncomingValuesToPhi(PHINode *PN,
   }
 }
 
+/// If the incoming values of a phi at \p UndefOps are a mix of undef and
+/// poison, replace the poison values with undef.
+///
+/// \param PN The phi whose incoming values are replaced.
+/// \param UndefOps The indices of incoming values that are undef or poison.
+static void replacePoisonWithUndefIfMixed(PHINode *PN,
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MaxBrandtner-IBM wrote:

I just optimized the existing logic. I'm fine either way, but I think it's probably better to offload that kind of behavior change to a separate PR.

https://github.com/llvm/llvm-project/pull/225647