AMDGPUAtomicOptimizer: re-enable uniform path for fadd/fsub with result

[jayfoad]

See #96479: the uniform path for fadd/fsub was disabled because it can return the wrong value in the first active lane of the result in the presence of NaNs and signed zeros.

We should find a way to fix and re-enable it, e.g. by overwriting the first active lane of %y * +0.0 with -0.0. And maybe this can be optimized more if certain values are known not to be zero or nan.

[llvmbot]

@llvm/issue-subscribers-backend-amdgpu

Author: Jay Foad (jayfoad)

See #96479: the uniform path for fadd/fsub was disabled because it can return the wrong value in the first active lane of the result in the presence of NaNs and signed zeros.

We should find a way to fix and re-enable it, e.g. by overwriting the first active lane of %y * +0.0 with -0.0. And maybe this can be optimized more if certain values are known not to be zero or nan.