[DAG][AArch64] Fold shuffle_vector<4,5,6,7> to extract_subvector

During legalization, we can end up with shuffles that are identity masks, so
act like extract_subvector, but do not simplify to extract_subvector. This
adjusts the profitability heuristic in foldExtractSubvectorFromShuffleVector to
allow identity vectors that do not start at element 0. Undef masks elements are
excluded as it can be more useful to keep the undef elements.

Differential Revision: https://reviews.llvm.org/D153504