Fix PR tree-optimization/71077

gcc/ChangeLog:

	PR tree-optimization/71077
	* tree-ssa-threadedge.c (simplify_control_stmt_condition_1): In
	the combining step, use boolean_false_node and boolean_true_node
	as the designated false/true return values.

gcc/testsuite/ChangeLog:

	PR tree-optimization/71077
	* gcc.dg/tree-ssa/pr71077.c: New test.

From-SVN: r236973