[AArch64][GlobalISel] Select @llvm.aarch64.neon.ld4.*

Reuse the selection code from the ld2 case. This is similar to how SDAG handles
things in AArch64ISelDAGToDAG. (See SelectLoad)

This fell back ~100 times while building clang with GISel enabled for AArch64.

Factoring out the gross subreg copy part ought to make selecting the rest of
this family fairly easy.

Differential Revision: https://reviews.llvm.org/D108600