[X86][SSE] Basic combining of unary target shuffles of binary target shuffles.

This patch reorders the combining of target shuffle masks so that when a unary shuffle takes a binary shuffle as its input but only references one of its inputs it can correctly combine into a unary shuffle mask.

This is starting to encroach on the purpose of resolveTargetShuffleInputs, but I don't want to remove it until we definitely know we won't need it for full binary shuffle combining.

There is a lot more work before we can properly support binary target shuffle masks but this was an easy case to add support for.

Differential Revision: http://reviews.llvm.org/D17858

llvm-svn: 263102