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.