Speedup DF dataflow solver

The following makes sure to process blocks that follow the current
block in the iteration order in the same iteration and only postpone
blocks that would be visited earlier to the next iteration.

For the all.i testcase in PR26854 at -O2 this shaves off 50% of
the time to solve the DF RD problem, other problems also improve
but not as drastically.

	PR middle-end/26854
	* df-core.cc (df_worklist_propagate_forward): Put later
	blocks on worklist and only earlier blocks on pending.
	(df_worklist_propagate_backward): Likewise.
	(df_worklist_dataflow_doublequeue): Change the iteration
	to process new blocks in the same iteration if that
	maintains the iteration order.

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