i965/fs: Make the first pre-allocation heuristic be the post heuristic.

i965/fs: Make the first pre-allocation heuristic be the post heuristic.

I recently made us try two different things that tried to reduce register
pressure so that we would be more likely to allocate successfully.  But
now that we have the logic for trying two, we can make the first thing we
try be the normal, not-prioritizing-register-pressure heuristic.

This means one less scheduling pass in the common case of that heuristic
not producing spills, plus the best schedule we know how to produce, if
that one happens to succeed.  This is important, because our register
allocation produces a lot of possibly avoidable dependencies for the
post-register-allocation schedule, despite ra_set_allocate_round_robin().

GLB2.7: 1.04127% +/- 0.732461% fps improvement (n=31)
nexuiz: No difference (n=5)
lightsmark: 0.838512% +/- 0.300147% fps improvement (n=86)
minecraft apitrace: No difference (n=15)

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>