[AArch64][SVE] Fix assertion failure when lowering fixed length gather/scatter

The patterns for fixed length gather/scatter with 32-bit offsets and
64-bit memory type are slightly different that the rest of the patterns,
as such the lowering needs to be slightly different to ensure the
correct types are used.

Differential Revision: https://reviews.llvm.org/D107576

(cherry picked from commit 73ecb998)