AMDGPU: Use tablegen patterns for buffer global and flat atomic fadd

Remove manual selection for atomic fadd from global-isel.
Stop pre-isel translation to AtomicLoadFAdd/G_ATOMICRMW_FADD
which corresponds to llvm-ir's atomicrmw fadd instruction.

global and flat atomic fadd patterns changes:
Split rtn/no-rtn patterns
Add missing patterns or fix predicates
Remove atomicrmw patterns for v2f16 (atomic rmw doesn't support vectors).
Patterns now check addrspace of pointer, added patterns for flat intrinsic.
with global addrspace pointer that selects into global atomic instruction.

buffer atomic fadd patterns changes:
Rdit patterns to import into global-isel.
Remove gfx6/gfx7 _addr64 and _offset patterns.
Remove patterns that can't be reached (same pattern but different feature).

Differential Revision: https://reviews.llvm.org/D130579