[llvm] [KnownBits] Make nuw and nsw support in computeForAddSub optimal (PR #83382)

[via llvm-commits]

goldsteinn wrote:

> LGTM.
> 
> Here's an idea for simplifying the code. I don't know if it's measurably faster. I don't really have time to finish it off at the moment. [jayfoad at c854484](https://github.com/jayfoad/llvm-project/commit/c854484260db11a9eebc79522d164661b43a4b33)

It is measurable faster :)
https://llvm-compile-time-tracker.com/compare.php?from=1f89a3cde2b407dc77e33ac3092133415ab151a9&to=9d52c0ffea77715ae70f1c66619504230d2da493&stat=instructions:u

https://github.com/llvm/llvm-project/pull/83382