[X86][AVX] Attempt to fold extract_subvector(shuffle(X)) -> extract_subvector(X)

If we're extracting a subvector from a shuffle that is shuffling entire subvectors we can peek through and extract the subvector from the shuffle source instead.

This helps remove some cases where concat_vectors(extract_subvector(),extract_subvector()) legalizations has resulted in BLEND/VPERM2F128 shuffles of the subvectors.