[x86] Tweak the ordering of unpack matching vs. element insertion, and

don't try to do element insertion for non-zero-index floating point
vectors.

We don't have any useful patterns or lowering for element insertion into
high elements of a floating point vector, and the generic shuffle
lowering will end up being better -- namely it will fall back to unpck.
But we should try to handle other forms of element insertion before
matching unpck patterns.

While this doesn't matter much right now, I'm working on a patch that
makes unpck matching much more powerful, and that patch will break
without this re-ordering.

llvm-svn: 229306