[x86] Delete still more piles of complex code now that we have a good

systematic lowering of v8i16.

This required a slight strategy shift to prefer unpack lowerings in more
places. While this isn't a cut-and-dry win in every case, it is in the
overwhelming majority. There are only a few places where the old
lowering would probably be a touch faster, and then only by a small
margin.

In some cases, this is yet another significant improvement.

llvm-svn: 229859