[X86] combineExtractSubvector - fold extract_subvector(insert_subvector(V,X,C1),C1)

extract_subvector(insert_subvector(V,X,C1),C1) -> insert_subvector(extract_subvector(V,C1),X,0)

More aggressively attempt to reduce the width of an extract_subvector source - we currently only do this if we're inserting into a zero vector (i.e. canonicalizing to the AVX implicit zero upper elts pattern).

But if we're extracting from the same point as the inner insert_subvector then the fold is still relatively trivial - we can probably do even better if we can ensure the subvector isn't badly split.