i965/sched: Calculate the critical path of scheduling nodes non-recursively.

The critical path of each node is calculated by induction based on the
critical paths of its children, which can be done in a post-order
depth-first traversal of the dependency graph.  The current code
implements graph traversal by iterating over all nodes of the graph
and then recursing into its children -- But it turns out that
recursion is unnecessary because the lexical order of instructions in
the block is already a good enough reverse post-order of the
dependency graph (if it weren't a reverse post-order some instruction
would have been located before one of its dependencies in the original
ordering of the basic block, which is impossible), so we just need to
walk the instruction list in reverse to achieve the same result more
efficiently.

No shader-db changes.

Reviewed-by: Jason Ekstrand <[email protected]>

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