tree-optimization/102155 - fix LIM fill_always_executed_in CFG walk

tree-optimization/102155 - fix LIM fill_always_executed_in CFG walk

This fixes the CFG walk order of fill_always_executed_in to use
RPO oder rather than the dominator based order computed by
get_loop_body_in_dom_order.  That fixes correctness issues with
unordered dominator children.

The RPO order computed by rev_post_order_and_mark_dfs_back_seme in
its for-iteration mode is a good match for the algorithm.

2021-09-01  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/102155
	* tree-ssa-loop-im.c (fill_always_executed_in_1): Iterate
	over a part of the RPO array and do not recurse here.
	Dump blocks marked as always executed.
	(fill_always_executed_in): Walk over the RPO array and
	process loops whose header we run into.
	(loop_invariant_motion_in_fun): Compute the first RPO
	using rev_post_order_and_mark_dfs_back_seme in iteration
	order and pass that to fill_always_executed_in.