[X86][AVX] combineExtractSubvector - 'little to big' extract_subvector(bitcast()) support

Ideally this needs to be a generic combine in DAGCombiner::visitEXTRACT_SUBVECTOR but there's some nasty regressions in aarch64 due to neon shuffles not handling bitcasts at all.....

llvm-svn: 364407