[X86][SSE] Shuffle combine blends to OR(X,Y) if the relevant elements are known zero.

This allows us to remove the (depth violating) code in getFauxShuffleMask where we were combining the OR(SHUFFLE,SHUFFLE) shuffle inputs as well, and not just the OR().

This is a minor step toward being able to shuffle combine from/to SELECT/BLENDV as a faux shuffle.