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

[via llvm-commits]

goldsteinn wrote:

> > > 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
> 
> Nice! How does that compare to main?

Improvement:
https://llvm-compile-time-tracker.com/compare.php?from=a4951eca40c070e020aa5d2689c08177fbeb780d&to=9d52c0ffea77715ae70f1c66619504230d2da493&stat=instructions%3Au

I bet it has to do with the `isUnknown` checks.

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