[AMDGPU][GlobalISel] Add support for global atomicrmw fadd

This includes gfx908 which only has a no-return version of the
global_atomic_add_f32 instruction, using the same hack that was
previously implemented for selecting from the
llvm.amdgcn.global.atomic.fadd intrinsic.

Differential Revision: https://reviews.llvm.org/D97767