[llvm] [NFC][AMDGPU] Move `flat_atomic.ll` to `llvm/test/CodeGen/AMDGPU/` (PR #141126)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

The test intent seems to have been lost; The names of the functions imply it's intended to test InferAddressSpaces running in the codegen pipeline 

https://github.com/llvm/llvm-project/pull/141126