AMDGPU/GlobalISel: Legalize f64 G_FFLOOR for SI

Use cmp ord instead of cmp_class compared to the DAG version for the
nan check, but mostly try to match the existsing pattern.

I think the sign doesn't matter for fract, so we could do a little
better with the source modifier matching.

I think this is also still broken as in D22898, but I'm leaving it
as-is for now while I don't have an SI system to test on.