[SVE][CodeGen] Improve codegen of scalable masked scatters

If the scatter store is able to perform the sign/zero extend of
its index, this is folded into the instruction with refineIndexType().
Additionally, refineUniformBase() will return the base pointer and index
from an add + splat_vector.

Reviewed By: sdesmalen

Differential Revision: https://reviews.llvm.org/D90942