[X86][SSE] When lowering a 256-bit shuffle as PMOVZX, reduce the input vector...

[X86][SSE] When lowering a 256-bit shuffle as PMOVZX, reduce the input vector to the lower 128-bit subvector.

Most often as not this is what it started out as, the extraction is zero-cost on AVX and the PMOVZX/PMOVSX folding logic is based around 128-bit loads.

llvm-svn: 270858