[llvm] [llvm] Optimize usub.sat fix for #79690 (PR #151044)

[Yingwei Zheng via llvm-commits]

https://github.com/dtcxzyw commented:

Can you please provide a generalized alive2 proof? See also https://llvm.org/docs/InstCombineContributorGuide.html#proofs

https://github.com/llvm/llvm-project/pull/151044