[llvm] [ConstraintElim] Simplify cmp after uadd.sat/usub.sat (PR #135603)

[Arthur Eubanks via llvm-commits]

aeubanks wrote:

I believe it's not UB if `%a >= 0` is not true, the later instructions just end up being poison

https://github.com/llvm/llvm-project/pull/135603