[x86] Factor out the zero vector insertion logic in the new vector
shuffle lowering for integer vectors and share it from v4i32, v8i16, and
v16i8 code paths.

Ironically, the SSE2 v16i8 code for this is now better than the SSSE3!
=] Will have to fix the SSSE3 code next to just using a single pshufb.