[llvm] Fold `icmp samesign u{gt/ge/lt/le} (X +nsw C2), C` -> `icmp s{gt/ge/lt/le} X, (C - C2)` (PR #169960)

[Tirthankar Mazumder via llvm-commits]

wermos wrote:

I'm not sure if I need to add more tests or not.

Ping @dtcxzyw for enabling CI and code review

https://github.com/llvm/llvm-project/pull/169960