8256999: Add C2 intrinsic for Reference.refersTo and PhantomReference::refersTo

Reviewed-by: pliden, vlivanov, rkennke, eosterlund, shade

src/java.base/share/classes/java/lang/ref/Reference.java

@@ -359,6 +359,7 @@ public abstract class Reference<T> {
 
     /* Implementation of refersTo(), overridden for phantom references.
      */
+    @IntrinsicCandidate
     native boolean refersTo0(Object o);
 
     /**