Virtualize relation oracle and various cleanups.

Standardize equiv_oracle API onto the new relation_oracle virtual base, and
then have dom_oracle inherit from that.
equiv_set always returns an equivalency set now, never NULL.
EQ_EXPR requires symmetry now.  Each SSA name must be in the other equiv set.
Shuffle some routines around, simplify.

	* gimple-range-cache.cc (ranger_cache::ranger_cache): Create a DOM
	based oracle.
	* gimple-range-fold.cc (fur_depend::register_relation): Use
	register_stmt/edge routines.
	* value-relation.cc (equiv_chain::find): Relocate from equiv_oracle.
	(equiv_oracle::equiv_oracle): Create self equivalence cache.
	(equiv_oracle::~equiv_oracle): Release same.
	(equiv_oracle::equiv_set): Return entry from self equiv cache if there
	are no equivalences.
	(equiv_oracle::find_equiv_block): Move list find to equiv_chain.
	(equiv_oracle::register_relation): Rename from register_equiv.
	(relation_chain_head::find_relation): Relocate from dom_oracle.
	(relation_oracle::register_stmt): New.
	(relation_oracle::register_edge): New.
	(dom_oracle::*): Rename from relation_oracle.
	(dom_oracle::register_relation): Adjust to call equiv_oracle.
	(dom_oracle::set_one_relation): Split from register_relation.
	(dom_oracle::register_transitives): Consolidate 2 methods.
	(dom_oracle::find_relation_block): Move core to relation_chain.
	(dom_oracle::query_relation): Rename from find_relation_dom and adjust.
	* value-relation.h (class relation_oracle): New pure virtual base.
	(class equiv_oracle): Inherit from relation_oracle and adjust.
	(class dom_oracle): Rename from old relation_oracle and adjust.