ZGC: Add on_weak load barrier verification

We've had a few occasions where the ZGC C2 code generated on_weak barriers instead of on_strong. Usually, those bugs goes unnoticed. The problem arise when a load barrier is applied to a reference with the finalizable bit set (reachable through finalizer), during the resurrection window.

I propose that we add some verification to ensure that on_weak barriers are only applied to java.lang.ref.Reference.referents.

This verification code catches the bugs in JDK-8225770 and JDK-8227083.