Verify unallocated edge/BB flags are clear

Verify unallocated edge/BB flags are clear

This adds verification that unused auto_{edge,bb}_flag are not
remaining set but correctly cleared by consumers.  The intent
is that those flags can be cheaply used on a smaller IL region
and thus afterwards clearing can be restricted to the same
small region as well.

2021-09-24  Richard Biener  <rguenther@suse.de>

	* cfghooks.c (verify_flow_info): Verify unallocated BB and
	edge flags are not set.