[X86][SSE] Fold vselect(pshufb,pshufb) -> or(pshufb,pshufb)

If the PSHUFBs have no other uses, then we can force the unselected elements to zero to OR them instead, avoiding both an extra mask load and a costly variable blend.

Eventually we should try to bring this into shuffle combining, once we can more easily convert between shuffles + select patterns.