r600/sb: deal with some scheduler problems
- Don't create three-source ops with all kcache values, it's unlikely that the used constants don't result in a kcache conflict
- fall back to un-optimized code when the scheduler fails. It is possible the optimizer created instructions that are impossible to schedule (e.g. MAX4(R0.x, R1.x, R2.x, R4.x), and fixing this in the sb copy propagation code seems quite difficult, so bail out in this case.