i965: Improve compute-to-mrf.

i965: Improve compute-to-mrf.

We were skipping it if the instruction producing the value we were
going to compute-to-mrf used its result reg as a source reg.  This
meant that the typical "write interpolated color to fragment color" or
"texture from interpolated texcoord" shader didn't compute-to-MRF.
Just don't check for the interference cases until after we've checked
if this is the instruction we wanted to compute-to-MRF.

Improves nexuiz high-settings performance on my laptop 0.48% +- 0.08%
(n=3).