[llvm] [InstCombine] 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:

> there is a fold above the new fold that handled this case without the samesign flag.

Are you suggesting that I should modify that code to handle the samesign case as well? Since that code uses `computeConstantRange` which I am not too familiar with, I opted to add a new if clause.

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