[llvm] [NFC][AMDGPU][Attributor] Move all pure AMDGPUAttributor related test files to `llvm/test/Transforms/Attributor/AMDGPU/` (PR #140815)

[Matt Arsenault via llvm-commits]

arsenm wrote:

Technically these aren't "attributor" tests, they are amdgpu-attributor which is a separate pass 

https://github.com/llvm/llvm-project/pull/140815