[AArch64][SelectionDAG] Support passing/returning scalable vectors with unusual types.

This adds handling for two cases:

1. A scalable vector where the element type is promoted.
2. A scalable vector where the element count is odd (or more generally,
   not divisble by the element count of the part type).

(Some element types still don't work; for example, <vscale x 2 x i128>,
or <vscale x 2 x fp128>.)

Differential Revision: https://reviews.llvm.org/D105591