Currently, for AMDGPU, when compiling for OpenCL, we unconditionally use
`private` as the default address space. This is wrong for cases where
the `generic` address space is available, and is corrected via this
patch. In general, this AS map abuse is a bad hack and we should re-work
it altogether, but at least after this patch we will stop being
incorrect for e.g. OpenCL 2.0.
We should have done this for the f32/f64 case a long time ago. Now that
codegen handles atomicrmw selection for the v2f16/v2bf16 case, start emitting
it instead.
This also does upgrade the behavior to respect a volatile qualified pointer,
which was previously ignored (for the cases that don't have an explicit
volatile argument).
