This adds handling for two cases:
- A scalable vector where the element type is promoted.
- 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>.)
The change to SplitVecRes_INSERT_SUBVECTOR needs to be considered in the context of the discussion of "widening" a scalable vector.