[llvm] [AMDGPU] Correct validation for the intended behavior of the test (PR #167411)

[via llvm-commits]

hidekisaito wrote:

> probably should switch to generated checks though

Thanks for the review @arsenm and @shiltian. Such a change should be done as NFC, but if we switch to auto-generated validation, I doubt many developers would be able to catch an incorrectly made automatic validation update. I'll keep it this way for human reviewers. At least, a comment is added to clearly state the intent.

https://github.com/llvm/llvm-project/pull/167411