[X86][AVX] Enable extraction of upper 128-bit subvectors for 'half undef' shuffle lowering

Added support for the extraction of the upper 128-bit subvectors for lower/upper half undef shuffles if it would reduce the number of extractions/insertions or avoid loads of AVX2 permps/permd shuffle masks.

Minor follow up to D15477.

llvm-svn: 258000