[InstCombine] Simplify select operand based on equality condition

For selects of the type X == Y ? A : B, check if we can simplify A
by using the X == Y equality and replace the operand if that's
possible. We already try to do this in InstSimplify, but will only
fold if the result of the simplification is the same as B, in which
case the select can be dropped entirely. Here the select will be
retained, just one operand simplified.

As we are performing an actual replacement here, we don't have
problems with refinement / poison values.

Differential Revision: https://reviews.llvm.org/D87480