[SelectionDAG] Make WidenVecRes_SELECT work for scalable vectors

    This change make WidenVecRes_SELECT work for scalable vectors.

    This patch is split from [D110319](https://reviews.llvm.org/D110319

)

Signed-off-by: Eric Tang <tangxingxin1008@gmail.com>

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D110388