[X86][SSE] Combine unary shuffle(HORIZOP,HORIZOP) -> HORIZOP

We had previously limited the shuffle(HORIZOP,HORIZOP) combine to binary shuffles, but we can often merge unary shuffles just as well, folding in UNDEF/ZERO values into the 64-bit half lanes.

For the (P)HADD/HSUB cases this is limited to fast-horizontal cases but PACKSS/PACKUS combines under all cases.