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

diff --git a/llvm/test/Transforms/InferAddressSpaces/AMDGPU/flat_atomic.ll b/llvm/test/CodeGen/AMDGPU/infer-addrspace-flat-atomic.ll
similarity index 100%
rename from llvm/test/Transforms/InferAddressSpaces/AMDGPU/flat_atomic.ll
rename to llvm/test/CodeGen/AMDGPU/infer-addrspace-flat-atomic.ll