[SelectionDAG] PromoteIntRes_EXTRACT_SUBVECTOR for scalable vectors.

This patch implements legalization of EXTRACT_SUBVECTOR for the case
where the result needs promoting, and the input type is either legal
or requires splitting.

The idea is that the operation is broken down into simpler steps,
by first extracting a smaller subvector until the input vector
becomes legal or requires promotion.

Reviewed By: CarolineConcatto

Differential Revision: https://reviews.llvm.org/D109313