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
