change DF to use the proper CFG order for DF_FORWARD problems

change DF to use the proper CFG order for DF_FORWARD problems

This changes DF to use RPO on the forward graph for DF_FORWARD
problems.  While that naturally maps to pre_and_rev_postorder_compute
we use the existing (wrong) CFG order for DF_BACKWARD problems
computed by post_order_compute since that provides the required
side-effect of deleting unreachable blocks.

The change requires turning the inconsistent vec<int> vs int * back
to consistent int *.  A followup patch will change the
inverted_post_order_compute API and change the DF_BACKWARD problem
to use the correct RPO on the backward graph together with statistics
I produced last year for the combined effect.

	* df.h (df_d::postorder_inverted): Change back to int *,
	clarify comments.
	* df-core.cc (rest_of_handle_df_finish): Adjust.
	(df_analyze_1): Likewise.
	(df_analyze): For DF_FORWARD problems use RPO on the forward
	graph.  Adjust.
	(loop_inverted_post_order_compute): Adjust API.
	(df_analyze_loop): Adjust.
	(df_get_n_blocks): Likewise.
	(df_get_postorder): Likewise.