[SSE/AVX] Use multiclasses to reduce the mass of scalar math patterns; NFCI

This takes the preposterous number of patterns in this section
that were last added to in r219033 down to just plain obnoxious.

With a little more work, we might get this down to just comical.

I've added more test cases to the existing file that checks these
patterns, but it seems that some of these patterns simply don't
exist with today's shuffle lowering.

llvm-svn: 229158