[x86] When splitting 256-bit vectors into 128-bit vectors, don't extract

subvectors from buildvectors. That doesn't really make any sense and it
breaks all of the down-stream matching of buildvectors to cleverly lower
shuffles.

With this, we now get the shift-based lowering of 256-bit vector
shuffles with AVX1 when we split them into 128-bit vectors. We also do
much better on the zero-extension patterns, although there remains quite
a bit of room for improvement here.

llvm-svn: 229299