[llvm] [IR] Remove zext and sext constant expressions (PR #71040)

[Nicolai Hähnle via llvm-commits]

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

Thank you for doing this. I looked through the code changes and a large random sample of the test changes, and they all look good to me except one spot where I'm confused -- and I suppose the change is fine as a matter of doing a mechanical change, but perhaps that code could be cleaned up further? Though if I'm right about it, it may be for a follow-up commit anyway.

https://github.com/llvm/llvm-project/pull/71040