[AArch64][GISel] Support SVE with 128-bit min-size for G_LOAD and G_STORE (#92130)

This patch adds basic support for scalable vector types in load & store
instructions for AArch64 with GISel.

Only scalable vector types with a 128-bit base size are supported, e.g.
`<vscale x 4 x i32>`, `<vscale x 16 x i8>`.

This patch adapted some ideas from a similar abandoned patch
[https://github.com/llvm/llvm-project/pull/72976](https://github.com/llvm/llvm-project/pull/72976).