[X86] Separate PentiumPro and i686. They aren't aliases in the backend.

PentiumPro has HasNOPL set in the backend. i686 does not.

Despite having a function that looks like it canonicalizes alias names. It
doesn't seem to be called. So I don't think this is a functional change. But its
good to be consistent between the backend and frontend.

llvm-svn: 356537