[amdgpu] Enhance AMDGPU AA.

- In general, a generic point may alias to pointers in all other address
  spaces. However, for certain cases enforced by the programming model,
  we may found a generic point won't alias to pointers to local objects.
  * When a generic pointer is loaded from the constant address space, it
    could only be a pointer to the GLOBAL or CONSTANT address space.
    Thus, it won't alias to pointers to the PRIVATE or LOCAL address
    space.
  * When a generic pointer is passed as a kernel argument, it also could
    only be a pointer to the GLOBAL or CONSTANT address space. Thus, it
    also won't alias to pointers to the PRIVATE or LOCAL address space.

Differential Revision: https://reviews.llvm.org/D89525