[X86][SSE] Allow target shuffle combining to match masks with SM_Sentinel values

We currently only allow exact matches of shuffle mask patterns during target shuffle combining.

This patch relaxes this to permit SM_SentinelUndef in the combined shuffle to always be accepted as well as allowing exact matching of the SM_SentinelZero value.

I've adjusted some tests that were requiring exact shuffle masks to now include undef values.

Differential Revision: http://reviews.llvm.org/D21495

llvm-svn: 273119