[SDAG] Teach the SelectionDAG to canonicalize vector shuffles of splats

directly into blends of the splats.

These patterns show up even very late in the vector shuffle lowering
where we don't have any chance for DAG combining to kick in, and
blending is a tremendously simpler operation to model. By coercing the
shuffle into a blend we can much more easily match and lower shuffles of
splats.

Immediately with this change there are significantly more blends being
matched in the x86 vector shuffle lowering.

llvm-svn: 229308