i965: Use is_scheduling_barrier instead of schedule_node::is_barrier.

i965: Use is_scheduling_barrier instead of schedule_node::is_barrier.

Commit a73116ecc60414ade89802150b tried to make add_barrier_deps()
walk to the next barrier, and stop.  To accomplish that, it added an
is_barrier flag.  Unfortunately, this only works half of the time.

The issue is that add_barrier_deps() walks both backward (to the
previous barrier), and forward (to the next barrier).  It also sets
is_barrier.  Assuming that we're processing instructions in forward
order, this means that is_barrier will be set for previous instructions,
but not future ones.  So we'll never see it, and walk further than we
need to.

dEQP-GLES31.functional.ssbo.layout.random.all_shared_buffer.23
now compiles its shaders in 3.6 seconds instead of 3.3 minutes.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Tested-by: Pallavi G <pallavi.g@intel.com>