Fix use of boolean_true/false_node (PR 83979)

Fix use of boolean_true/false_node (PR 83979)

r255913 changed some constant_boolean_node calls to boolean_true_node
and boolean_false_node, which meant that the returned tree didn't
always have the right type.

2018-01-24  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR tree-optimization/83979
	* fold-const.c (fold_comparison): Use constant_boolean_node
	instead of boolean_{true,false}_node.

gcc/testsuite/
	PR tree-optimization/83979
	* g++.dg/pr83979.c: New test.

From-SVN: r257021