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

[Tirthankar Mazumder via llvm-commits]

wermos wrote:

I haven't handled the nuw + nsw case yet, I'll do that later because I am traveling right now.

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