AMDGPU: Refactor atomicrmw fadd expansion logic (NFC) (#89469)

This had some repeated and overlapping conditions, which
made it more difficult to handle the new metadata scheme. Reflow
the function to handle the easy LDS cases first. For the flat/global
cases, write in a positive-enabled style where everything unhandled
hits a default cmpxchg.

Depends #89468