[ARM] Extend narrow values to allow using truncating scatters

As a minor adjustment to the existing lowering of offset scatters, this
extends any smaller-than-legal vectors into full vectors using a zext,
so that the truncating scatters can be used. Due to the way MVE
legalizes the vectors this should be cheap in most situations, and will
prevent the vector from being scalarized.

Differential Revision: https://reviews.llvm.org/D103704