[AArch64][GISel] Lower llvm.prefetch

This change adds support for lowering llvm.prefetch directly using
GlobalISel. Currently, llvm.prefetch falls back to SelectionDAG.

This Change:
- Adds an AArch64-specific G_PREFETCH generic instruction, to be used
  where AArch64ISD::PREFETCH is used in SelectionDAG.
- Adds the GINodeEquiv so patterns are translated over to GlobalISel
  automatically.
- Corrects the AArch64Prefetch patterns to use a target immediate, which
  is needed to get the patterns to translate across correctly.
- Translates the SelectionDAG legalisation of the prefetch intrinsic
  into the corresponding GlobalISel legalisation.

Differential Revision: https://reviews.llvm.org/D132043