[llvm] [SimplifyCFG] Improve FoldTwoEntryPHINode when one of phi values is undef (PR #69021)

[Nikita Popov via llvm-commits]

https://github.com/nikic requested changes to this pull request.

You are not allowed to use the DominatorTree inside SimplifyCFG, only to preserve it.

https://github.com/llvm/llvm-project/pull/69021