On nv50, certain operations must happen on regs below 64, due to
encoding requirements. First of all, we add infrastructure to enforce
this. Secondly we change the spill order to first spill RIG nodes that
are unconstrained, followed by ones that are.

This makes the gamecube logo shadertoy compile properly. Curiously, if
we adjust the spill order so that we first spill the constrained RIG
nodes instead, the RA also succeeds. However it seems more logical to
first spill the unconstrained ones.

While we're at it, drop the nv50 max register to reserve r127 as the
zero register of last resort (r63 is preferred).

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Acked-by: Karol Herbst <kherbst@redhat.com>