[llvm] [InstCombine] Improve coverage of `foldSelectValueEquivalence` (PR #88298)

[via llvm-commits]

goldsteinn wrote:

@nikic and @dtcxzyw

I simplified the code a bit. Hopefully that helps. Also I split changes for folding constants / non-constants into two seperate commits.

If its still too complicated I will split to 2 seperate PRs. That said I hope for both of the changes to get in. The non-const change seems to be the more common in @dtcxzyw's tests and seems to have independent value.

https://github.com/llvm/llvm-project/pull/88298