[AArch64] Support SLC in ACLE prefetch intrinsics

This change:
- Modifies the ACLE code to allow the new SLC value (3) for the prefetch
  target.

- Introduces a new intrinsic, @llvm.aarch64.prefetch which matches the
  PRFM family instructions much more closely, and can represent all
  values for the PRFM immediate.

  The target-independent @llvm.prefetch intrinsic does not have enough
  information for us to be able to lower to it from the ACLE intrinsics
  correctly.

- Lowers the acle calls to the new intrinsic on aarch64 (the ARM
  lowering is unchanged).

- Implements code generation for the new intrinsic in both SelectionDAG
  and GlobalISel. We specifically choose to continue to support lowering
  the target-independent @llvm.prefetch intrinsic so that other
  frontends can continue to use it.

Differential Revision: https://reviews.llvm.org/D139443