Improve var-tracking dataflow iteration order

Improve var-tracking dataflow iteration order

This builds upon the rev_post_order_and_mark_dfs_back_seme improvements
and makes vt_find_locations iterate over the dataflow problems for
each toplevel SCC separately, improving memory locality and avoiding
to process nodes after the SCC before the SCC itself stabilized.

On the asan_interceptors.cc testcase this for example reduces the
number of visited blocks from 3751 to 2867.  For stage3-gcc
this reduces the number of visited blocks by ~4%.

2020-07-28  Richard Biener  <rguenther@suse.de>

	PR debug/78288
	* var-tracking.c (vt_find_locations): Use
	rev_post_order_and_mark_dfs_back_seme and separately iterate
	over toplevel SCCs.