[X86][SSE] Improved support for decoding target shuffle masks through bitcasts

Reused the ability to split constants of a type wider than the shuffle mask to work with masks generated from scalar constants transfered to xmm.

This fixes an issue preventing PSHUFB target shuffle masks decoding rematerialized scalar constants and also exposes the XOP VPPERM bug described in PR27472.

llvm-svn: 267343