[AMDGPU] Add test for a problem with noclobber metadata

If AMDGPUAnnotateUniformValues finds a load from a uniform pointer with
no potentially clobbering stores between the kernel entry point and the
load instruction, it adds noclobber metadata to the *address*. This is
unsafe because it can get applied to other loads in the same which do
have aliasing stores.

Differential Revision: https://reviews.llvm.org/D118458