[X86][SSE] Improvements to byte shift shuffle matching

Since (v)pslldq / (v)psrldq instructions resolve to a single input argument it is useful to match it much earlier than we currently do - this prevents more complicated shuffles (notably insertion into a zero vector) matching before it.

Differential Revision: http://reviews.llvm.org/D6409



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222796 91177308-0d34-0410-b5e6-96231b3b80d8