[AArch64][GlobalISel] Select llvm.aarch64.neon.st2 intrinsics

Add manual selection code similar to the code in AArch64ISelDAGToDAG, and add
`createTuple` helpers similar to the code there as well.

This accounted for around 111 fallbacks while building clang for AArch64 with
GlobalISel.

This also should make it easy to add selection code for other store
intrinsics.

As a minor cleanup, this uses `createQTuple` in the other place where we use
REG_SEQUENCE.

Differential Revision: https://reviews.llvm.org/D106332