[llvm] [InstCombine] Simplify select using KnownBits of condition (PR #95923)

[Nikita Popov via llvm-commits]

nikic wrote:

> > It doesn't fully avoid the regression on that test case, as many selects do have potentially affected values, but at least cuts it in half.
> > Do you want me to include that?
> 
> Unless it dramatically undermines the value of the patch, halfway is better than nothing no?

It shouldn't affect the results at all. I went ahead and added it.

Here are the final results for this PR: http://llvm-compile-time-tracker.com/compare.php?from=656b8f5ec4ba3fe8ec7bdef125ccd42ed43b0b16&to=14ad6079cba73d37eb8997f8f3bf576e0fd21f2a&stat=instructions%3Au

https://github.com/llvm/llvm-project/pull/95923