i965/sched: Assign a preferred exit node to each node of the dependency graph.

i965/sched: Assign a preferred exit node to each node of the dependency graph.

This adds a bit of metadata to schedule_node that will be used to
compare available nodes in the scheduling heuristic code based on
which of them unblocks the earliest successor exit node.  Note that
assigning exit nodes wouldn't be necessary in a bottom-up scheduler
because we could achieve the same effect by scheduling the exit nodes
themselves appropriately.

No shader-db changes.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>