[AArch64][GlobalISel] Select @llvm.aarch64.neon.ld2.*

This is pretty similar to the ST2 selection code in
`AArch64InstructionSelector::selectIntrinsicWithSideEffects`.

This is a GISel equivalent of the ld2 case in `AArch64DAGToDAGISel::Select`.
There's some weirdness there that appears here too (e.g. using ld1 for scalar
cases, which are 1-element vectors in SDAG.)

It's a little gross that we have to create the copy and then select it right
after, but I think we'd need to refactor the existing copy selection code
quite a bit to do better.

This was falling back while building llvm-project with GISel for AArch64.

Differential Revision: https://reviews.llvm.org/D108590