[AMDGPU] Check atomics aliasing in the clobbering annotation

MemorySSA considers any atomic a def to any operation it dominates
just like a barrier or fence. That is correct from memory state
perspective, but not required for the no-clobber metadata since
we are not using it for reordering. Skip such atomics during the
scan just like a barrier if it does not alias with the load.

Differential Revision: https://reviews.llvm.org/D118661