[llvm] [InstCombine] optimize unnecessary sext instruction with add + cmp (PR #152291)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

Please see llvm.org/docs/InstCombineContributorGuide.html. Please add an alive2 proof to the PR description.

https://github.com/llvm/llvm-project/pull/152291