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

[Nikita Popov via llvm-commits]

nikic wrote:

We could also simplify usub.sat to sub if LHS >= RHS. This patch just adds fact, but we could also simplify.

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