Add equivalence iterator to relation oracle.

Add equivalence iterator to relation oracle.

Instead of looping over an exposed equivalence bitmap, provide iterators
to loop over equivalences, partial equivalences, or both.

	* gimple-range-cache.cc (ranger_cache::fill_block_cache): Use
	iterator.
	* value-relation.cc
	  (equiv_relation_iterator::equiv_relation_iterator): New.
	(equiv_relation_iterator::next): New.
	(equiv_relation_iterator::get_name): New.
	* value-relation.h (class relation_oracle): Privatize some methods.
	(class equiv_relation_iterator): New.
	(FOR_EACH_EQUIVALENCE): New.
	(FOR_EACH_PARTIAL_EQUIV): New.
	(FOR_EACH_PARTIAL_AND_FULL_EQUIV): New.