i965: Generalize VS compute-to-MRF for compute-to-another-GRF, too.

i965: Generalize VS compute-to-MRF for compute-to-another-GRF, too.

No statistically significant performance difference on glbenchmark 2.7
(n=60).  It reduces cycles spent in the vertex shader by 3.3% +/- 0.8%
(n=5), but that's only about .3% of all cycles spent according to the
fixed shader_time.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>