[AArch64][SVE] Add support for fixed length MSCATTER/MGATHER

Since gather lowering can now lower to nodes that may need expansion via
the vector legalizer, do MGATHER lowering via vector legalizer.

Additionally, as part of adding passthru support for fixed typed
gathers, fix passthru support for scalable types.

Depends on D104910

Differential Revision: https://reviews.llvm.org/D104217