Noticed this when working on JDK-8376749. When Verifier is about to fail on broken oop, it would decode the broken compressed pointer before doing any verification on it. So the end result is that we _at best_ catch the non-verbose assert in shared code in debug builds, or we crash the VM with opaque error in release builds, instead of rich Shenandoah verification failure.

We need to tighten this up, so that Verifier does work properly in these conditions.