AMDGPU/GlobalISel: Manually select scalar f64 G_FNEG

This should be no problem to support with a pattern, but it turns out
there are just too many yaks to shave. The main problem is in the DAG
emitter, which I have no desire to sink effort into fixing.

If we had a bit to disable patterns in the DAG importer, fixing the
GlobalISelEmitter is more manageable.