[AMDGPU] Fix scan of atomicFSub in AtomicOptimizer. (#66082)

[D156301](https://reviews.llvm.org/D156301

) introduced atomic
optimizations for FAdd/FSub. For FSub, reduction/scan needs to be
performed using add operation (`not sub`) and memory location will be
updated by reduced value using atomic sub later by only one lane.

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Authored-by: Pravin Jagtap <Pravin.Jagtap@amd.com>