v3d: Rotate through registers to improve post-RA scheduling options.

v3d: Rotate through registers to improve post-RA scheduling options.

Similarly to VC4's implementation, by not picking r0 immediately upon
freeing it, we give the scheduler more of a chance to fit later writes in
earlier.  I'm not clear on whether there's any real cost to picking phys
over accumulators, so keep that behavior for now.

shader-db:
total instructions in shared programs: 96831 -> 95669 (-1.20%)
instructions in affected programs:     77254 -> 76092 (-1.50%)