make var-tracking iteration consistent

make var-tracking iteration consistent

This eliminates the visited bitmap and makes whether a to be processed
block goes to the next or the current iteration only depend on its
position in RPO order rather than on whether it was visited in the
current iteration.  As optimization single-BB iteration is processed
immediately.

2020-07-10  Richard Biener  <rguenther@suse.de>

	* var-tracking.c (bb_heap_node_t): Remove unused typedef.
	(vt_find_locations): Eliminate visited bitmap in favor of
	RPO order check.  Dump statistics about the number of
	local BB dataflow computes.