This is the first step towards refactoring the x86 vector shuffle code. The

general idea here is to have a group of x86 target specific nodes which are
going to be selected during lowering and then directly matched in isel.

The commit includes the addition of those specific nodes and a *bunch* of
patterns, and incrementally we're going to switch between them and what we
have right now. Both the patterns and target specific nodes can change as
we move forward with this work.

llvm-svn: 111691