Add special test for generic address space atomics

[silverclaw]

The atomic operations are tested with generic pointers but in a way where the compiler can infer the original pointer address space. This commit adds tests specifically for the case where the compiler cannot make inferences. Test that the correct address is used with atomics when the address is group variant and invariant.

[lakshmih]

Reviewing and will have some feedback next week.

[silverclaw]

Updated for formatting fixes, and added a requested comment explaining why the PREFERRED workgroup size is being queried.

[bashbaug]

Merging as discussed in the June 25th teleconference.