[X86] Add custom isel logic to match VPTERNLOG from 2 logic ops.

There's room from improvement here, but this is a decent
starting point.

There are a few minor regressions in the vector-rotate tests,
where we are now forming a vpternlog from an and before we get
a chance to form it for a bitselect that we were matching
previously. This results in an AND and an ANDN feeding the
vpternlog where previously we just had an AND after the
vpternlog. I think we can probably DAG combine the AND with
the bitselect to get back to similar codegen.

llvm-svn: 373172

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