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

[Jay Foad via llvm-commits]

https://github.com/jayfoad approved this pull request.

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.
https://github.com/jayfoad/llvm-project/commit/c854484260db11a9eebc79522d164661b43a4b33

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