AMDGPU: Improve atomicrmw fadd selection

Use same atomicrmw fadd expansion rules for gfx908, gfx940 and gfx11
as for gfx90a. Add missing globalisel legalizer support for flat
atomicrmw fadd f32 on gfx940 and gfx11.
Isel support for gfx11 will be added in D130579.

Differential Revision: https://reviews.llvm.org/D131560