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

[Nikita Popov via llvm-commits]

nikic wrote:

See select_replacement_add_eq_vec_undef for the problematic case. The issue is with undef values, not poison values, and that problem exists even if the result is a constant. I've clarified the implementation to that effect.

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