tree+ggc: Change return type of predicate functions from int to bool

Also change internal variable from int to bool.

gcc/ChangeLog:

	* tree.h (tree_int_cst_equal): Change return type from int to bool.
	(operand_equal_for_phi_arg_p): Ditto.
	(tree_map_base_marked_p): Ditto.
	* tree.cc (contains_placeholder_p): Update function body
	for bool return type.
	(type_cache_hasher::equal): Ditto.
	(tree_map_base_hash): Change return type
	from int to void and adjust function body accordingly.
	(tree_int_cst_equal): Ditto.
	(operand_equal_for_phi_arg_p): Ditto.
	(get_narrower): Change "first" variable to bool.
	(cl_option_hasher::equal): Update function body for bool return type.
	* ggc.h (ggc_set_mark): Change return type from int to bool.
	(ggc_marked_p): Ditto.
	* ggc-page.cc (gt_ggc_mx): Change return type
	from int to void and adjust function body accordingly.
	(ggc_set_mark): Ditto.