freedreno/ir3: add experimental GCM pass

freedreno/ir3: add experimental GCM pass

Generally seems to do worse on instruction count and register usage,
according to shader-db.  But shader-db also doesn't do a very good job
of weighting loop bodies, so that might not be totally valid.

So add an env variable to enable GCM pass for easier experimentation.

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