freedreno/ir3: track register pressure in sched

freedreno/ir3: track register pressure in sched

Not a perfect solution, and the "pressure" target is hard-coded.  But it
doesn't really seem to much in the common case, and avoids exploding
register usage in dEQP ssbo tests.

So this should serve as a stop-gap solution until I have time to re-
write the scheduler.

Hurts slightly in instruction count, but gains (reduces) slightly the
register usage in shader-db.  Fixes ~150 dEQP-GLES31.functional.ssbo.*
that were failing due to RA fail.

Signed-off-by: Rob Clark <robdclark@gmail.com>