[llvm] [InstCombine] Fold `switch(zext/sext(X))` into `switch(X)` (PR #76988)

[Nikita Popov via llvm-commits]

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

LGTM if you're happy with the diffs :)

https://github.com/llvm/llvm-project/pull/76988