[SelectionDAG] Propagate scoped AA metadata when lowering mem intrinsics.
- When memory intrinsics, such as memcpy, the attached scoped AA metadata is not passed down to the backend. As a result, the backend cannot schedule relevant memory operations around them following that hint. In this patch, SelectionDAG is enhanced to propagate that metadata (scoped AA only) when they are lowered into loads and stores.
Differential Revision: https://reviews.llvm.org/D102215