[x86] Add the beginnings of a proper DAG combine to match ADDSUBPS and

ADDSUBPD nodes out of blends of adds and subs.

This allows us to actually form these instructions with SSE3 rather than
only forming them when we had both SSE3 for the ADDSUB instructions and
SSE4.1 for the blend instructions. ;] Kind-of important.

I've adjusted the CPU requirements on one of the tests to demonstrate
this kicking in nicely for an SSE3 cpu configuration.

llvm-svn: 217848