re PR tree-optimization/46590 (long compile time with -O2 and many loops)

re PR tree-optimization/46590 (long compile time with -O2 and many loops)

2019-04-01  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/46590
	* domwalk.h (dom_walker::dom_walker): Consolidate constructors.
	(dom_walker::m_reachability): Add in place of...
	(dom_walker::m_skip_unreachable_blocks): ...this.
	* domwalk.c (dom_walker::dom_walker): Consoliate constructors.
	Move complex initialization ...
	(dom_walker::walk): Here.  Especially compute m_bb_to_rpo
	lazily and initialize edge flags on each invocation.
	(dom_walker::bb_reachable): Use m_reachability.

From-SVN: r270055