[X86] Genericize the handling of INSERT_SUBVECTOR from an EXTRACT_SUBVECTOR to...

[X86] Genericize the handling of INSERT_SUBVECTOR from an EXTRACT_SUBVECTOR to support 512-bit vectors with 128-bit or 256-bit subvectors.

We now detect that both the extract and insert indices are non-zero and convert to a shuffle. This will be lowered as a blend for 256-bit vectors or as a vshuf operations for 512-bit vectors.

llvm-svn: 294931