[llvm] [CGP] Fold cmp+br+sub to usub+br on the overflow flag (PR #226496)

[Hans Wennborg via llvm-commits]

zmodem wrote:

We already have the machinery to do this; https://github.com/llvm/llvm-project/pull/227681 is a better version of this PR.

https://github.com/llvm/llvm-project/pull/226496