[llvm] [SimplifyCFG] Replace unreachable switch lookup table holes with poison (PR #94990)

[via llvm-commits]

DaMatrix wrote:

Okay, this new commit will still fail that check as there are (necessarily) new occurrences of `undef` in the tests, but the actual code no longer contains any new references to `undef` (except for in comments).

https://github.com/llvm/llvm-project/pull/94990