alias: Change return type of predicate functions from int to bool

Also remove a bunch of unneeded forward declarations.

gcc/ChangeLog:

	* rtl.h (true_dependence): Change return type from int to bool.
	(canon_true_dependence): Ditto.
	(read_dependence): Ditto.
	(anti_dependence): Ditto.
	(canon_anti_dependence): Ditto.
	(output_dependence): Ditto.
	(canon_output_dependence): Ditto.
	(may_alias_p): Ditto.
	* alias.h (alias_sets_conflict_p): Ditto.
	(alias_sets_must_conflict_p): Ditto.
	(objects_must_conflict_p): Ditto.
	(nonoverlapping_memrefs_p): Ditto.
	* alias.cc (rtx_equal_for_memref_p): Remove forward declaration.
	(record_set): Ditto.
	(base_alias_check): Ditto.
	(find_base_value): Ditto.
	(mems_in_disjoint_alias_sets_p): Ditto.
	(get_alias_set_entry): Ditto.
	(decl_for_component_ref): Ditto.
	(write_dependence_p): Ditto.
	(memory_modified_1): Ditto.
	(mems_in_disjoint_alias_set_p): Change return type from int to bool
	and adjust function body accordingly.
	(alias_sets_conflict_p): Ditto.
	(alias_sets_must_conflict_p): Ditto.
	(objects_must_conflict_p): Ditto.
	(rtx_equal_for_memref_p): Ditto.
	(base_alias_check): Ditto.
	(read_dependence): Ditto.
	(nonoverlapping_memrefs_p): Ditto.
	(true_dependence_1): Ditto.
	(true_dependence): Ditto.
	(canon_true_dependence): Ditto.
	(write_dependence_p): Ditto.
	(anti_dependence): Ditto.
	(canon_anti_dependence): Ditto.
	(output_dependence): Ditto.
	(canon_output_dependence): Ditto.
	(may_alias_p): Ditto.
	(init_alias_analysis): Change "changed" variable to bool.