AMDGPU: Simplify DS atomicrmw fadd handling (#89468)

DS atomic fadd F32 does respect the denormal mode, so we do not need to
consider the expected FP mode or unsafe-fp-atomics attribute. They don't
respect the rounding mode, but we don't care outside of strictfp. This
also reveals the fp-mode-is-flush check has been missing in the cases
that should be considering it alongside amdgpu-unsafe-fp-atomics.

This also stops considering the case where flushing is enabled for f64,
as flushing isn't mandated and we barely handle this case.