JVM_ReferenceRefersTo should only be used for soft/weak references and not phantom references, which has its own implementation. This is not clear from the name. `java_lang_ref_Reference::weak_referent_no_keepalive(ref_oop)` is only a valid call if `!java_lang_ref_Reference::is_phantom(ref_oop)`.

Adds an assert and comment to make this clear.