[llvm] [InstCombine] Add optimization to combine adds through zext nneg, and add testcase (PR #157723)

[Nikita Popov via llvm-commits]

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

> I believe this is a translation of your preconditions: https://alive2.llvm.org/ce/z/nrK53A It shows that they are not correct. Even if the addition result is a signed i8, the fact that the original constant is larger is a problem.
>
> This variant works: https://alive2.llvm.org/ce/z/5geVhA

This issue is still open. To give a specific example, you are currently miscompiling this test case: https://alive2.llvm.org/ce/z/66TJWX

https://github.com/llvm/llvm-project/pull/157723