Combine vextract (v1iX extract_subvector(vNiX, Idx)) into vextract(vNiX,Idx).
This case appeared in AVX512 after fixing pr33349 in r310552.
Usually we don't get these cases since v1iX are illegal in most targets and then these
(v1iX extract_subvector(vNiX, Idx)) get scalarized into (iX vextract(vNiX,Idx)).
But for targets like AVX512 where such type is legal we end up with a redundant DAG
node which can be combined.