[AArch64][SVE] Change pointer type of struct load/store intrinsics.

Instead of loading from e.g. `<vscale x 16 x i8>*`, load from element
pointer `i8*`. This is more in line with the other load/store
intrinsics for SVE.

Reviewers: fpetrogalli, c-rhodes, rengolin, efriedma

Reviewed By: efriedma

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81458