[X86] Use 128-bit ops for 256-bit vzmovl patterns.

128-bit ops implicitly zero the upper bits. This should address the comment about domain crossing for the integer version without AVX2 since we can use a 128-bit VBLENDW without AVX2.

The only bad thing I see here is that we failed to reuse an vxorps in some of the tests, but I think that's already known issue.

llvm-svn: 337134