[X86][SSE] Add extract_subvector(PSHUFB) -> PSHUFB(extract_subvector()) combine

Fixes PR32160 by reducing the size of PSHUFB if we only use one of the lanes.

This approach can probably be generalized to handle any target shuffle (and any subvector index) but we have no test coverage at the moment.

llvm-svn: 344336