AMDGPU: Handle legal v2f16/v2bf16 atomicrmw fadd for global/flat

Unlike the existing fadd cases, choose to ignore the requirement for
amdgpu-unsafe-fp-atomics in case of fine-grained memory access. This
is to minimize migration pain to the new atomic control metadata. This
should not break any users, as the atomic intrinsics are still
directly consumed, and clang does not yet produce vector FP atomicrmw.