[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