[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