[llvm] Reland "[SimplifyCFG] Check if the return instruction causes undefined behavior" (PR #76656)

[Nikita Popov via llvm-commits]

https://github.com/nikic approved this pull request.

LGTM, but please wait until after LLVM 18 has branched to merge it. Please also perform the merge manually to avoid squashing the commits.

https://github.com/llvm/llvm-project/pull/76656