Fix LCM dataflow CFG order

Fix LCM dataflow CFG order

The following fixes the initial order the LCM dataflow routines process
BBs.  For a forward problem you want reverse postorder, for a backward
problem you want reverse postorder on the inverted graph.

The LCM iteration has very many other issues but this allows to
turn inverted_post_order_compute into computing a reverse postorder
more easily.

	* lcm.cc (compute_antinout_edge): Use RPO on the inverted graph.
	(compute_laterin): Use RPO.
	(compute_available): Likewise.