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

[Matt Arsenault via llvm-commits]

https://github.com/arsenm approved this pull request.

We'll probably need to change this to run opt on the IR and then llc later 

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