[X86][SSE] Added common helper for shuffle mask constant pool decodes.

[X86][SSE] Added common helper for shuffle mask constant pool decodes.

The shuffle mask decodes have a large amount of repeated code extracting/splitting mask values from Constant data.

This patch pulls all of this duplicated code into a single helper function to identify undef elements and combine/split constant integer data into the requested shuffle mask elements.

Updated PSHUFB/VPERMIL/VPERMIL2/VPPERM decoders to use it (VPERMV/VPERMV3 could be converted as well in the future).

llvm-svn: 282720