tree-ssa-dom.c (optimize_stmt): Make this a method within the dom_opt_dom_walker class with direct access to...

	* tree-ssa-dom.c (optimize_stmt): Make this a method within the
	dom_opt_dom_walker class with direct access to private members.
	Add comments.  Call test_for_singularity.
	(dom_opt_dom_walker::before_dom_children): Corresponding changes.
	(dom_opt_dom_walker::after_dom_children): Do not lazily initialize
	m_dummy_cond anymore.
	(class dom_opt_dom_walker): Initialize m_dummy_cond member in the
	class ctor.
	(pass_dominator:execute): Build the dummy_cond here and pass it
	to the dom_opt_dom_walker ctor.
	(test_for_singularity): New function.

	* gcc.dg/tree-ssa/ssa-dom-simplify-1.c: New test.

2017-09-30  Paolo Carlini  <paolo.carlini@oracle.com>

From-SVN: r253329