[X86] Combine concat(shufps,shufps) -> shufps(concat,concat)

Now that rG18c19441d105 has improved VPERM2X128 handling, we can perform this to improve x64->x32 truncation without poor cross-lane issues.

Someday combineX86ShufflesRecursively will handle this, but we're still really bad at dealing with different vector widths.