[AMDGPU] Only allow global fp atomics with unsafe option

Previously we were allowing to use FP atomics without
-amdgpu-unsafe-fp-atomics option if a scope is less then
system. This is not safe just as well if we have UC memory.

This change only allows global and flat FP atomics with
the unsafe option. Consequentially that makes a check for
denorm mode redundant since we skip it with the unsafe
option and do not have a way to produce these instructions
without it anyway.

Differential Revision: https://reviews.llvm.org/D102347