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

[via llvm-commits]

Aethezz wrote:

> > Updated proof: https://alive2.llvm.org/ce/z/T2BmZj
> 
> This proof is not correct? src and tgt are the same here.
> 
> 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

Oh thanks, I didn't export the updated link after editing the proof.

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