[AArch64][SelectionDAG] Refactor to support more scalable vector extending stores

Similar to D122281, we should firstly exclude all scalable vector extending
stores and then selectively enable those which we directly support.

Also merge integer and float scalable vector into scalable_vector_valuetypes.

Reviewed By: paulwalker-arm

Differential Revision: https://reviews.llvm.org/D123449