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

[Max Brandtner via llvm-commits]

MaxBrandtner-IBM wrote:

Both of the undef value hits are expected here:

local.cpp: I moved the occurrence of UndefValue::get from replaceUndefValuesInPhi into replacePoisonWithUndefIfMixed.

poison-merge.ll: added a test to check that poison values are converted to undef when merging the phi nodes of a folded block if there are poison and undef values.

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