[SelectionDAG] Make WidenVecRes_Convert work for scalable vectors.

Most of the code wasn't yet scalable safe, although most of the
code conceptually just works for scalable vectors. This change
makes the algorithm work on ElementCount, where appropriate,
and leaves the fixed-width only code to use `getFixedNumElements`.

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D110058