[AArch64][SVE] Fix incorrect mask type when lowering fixed type SVE gather/scatter

An incorrect mask type when lowering an SVE gather/scatter was causing
a codegen fault which manifested as the incorrect predicate size being
used for an SVE gather/scatter, (e.g.. p0.b rather than p0.d).

Fixes PR51182.

Differential Revision: https://reviews.llvm.org/D106943