[X86] Lower blended PACKUSes using appropriate types.

When lowering two blended PACKUS, we used to disregard the types
of the PACKUS inputs, indiscriminately generating a v16i8 PACKUS.

This leads to non-selectable things like:
    (v16i8 (PACKUS (v4i32 v0), (v4i32 v1)))

Instead, check that the PACKUSes have the same type, and use that
as the final result type.

llvm-svn: 274138