Fix spurious valgrind errors in irred loop verification

Fix spurious valgrind errors in irred loop verification

The sbitmap bitmap_{set,clear}_bit changes trigger spurious
uninit value use reportings from valgrind since we now
read the old value before setting/clearing a bit so
verify_loop_structures optimization to not clear the sbitmap is reported.

Fixed by using a temporary BB flag which should also be more
efficient in terms of cache re-use.

2021-11-08  Richard Biener  <rguenther@suse.de>

	* cfgloop.c (verify_loop_structure): Use a temporary BB flag
	instead of an sbitmap to cache irreducible state.