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

[Alexander Richardson via llvm-commits]

arichardson wrote:

> Annoying amount of duplication, we could probably just drop the r600 run here

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

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