[llvm] [AtomicExpand] Drop explicit datalayout from test (PR #137923)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I'm happy with either of those solutions, please let me know which one you prefer.

Either way, I guess drop the r600 checks. Theoretically this should be covered under codegen tests, but I'm not sure r600 has coverage there (I'd be surprised if all the atomics worked there in general) 



https://github.com/llvm/llvm-project/pull/137923