R600/SI: Fix/add tests for atomicrmw add

The previous tests claimed to test constant offsets in the function name,
but the tests weren't actually testing them.

Clone the tests, and do testing of all combinations of the following:
1) with/without constant pointer offset
2) 32/64-bit addressing modes
3) Usage and non-usage of the return value from the atomicrmw

Reviewed-by: Matt Arsenault <matthew.arsenault@amd.com>
llvm-svn: 220103