i965/fs: Assign URB/CURB register numbers after instruction scheduling.

i965/fs: Assign URB/CURB register numbers after instruction scheduling.

This fixes a bunch of unnecessary barriers due to the scheduler not
knowing what that arbitrary register description refers to when trying
to reason about its dependencies.

The result is rescheduling in the convolution kernel shader in
Lightsmark, which results in avoiding register spilling and increasing
the performance of the first scene from 6-7 fps midway through the
panning to 11fps.  The register spilling was a regression from Mesa
7.9 to Mesa 7.10.