In JDK-8309764 shows a failure in heap verification (during VM exit) where a decoded oop is not in the heap.

It uses CompressedOops::decode_not_null() which does its own in-heap checking, which is unfortunate as it reveals little information about the object that the subsequent code in heap verification would have printed (and then exited the VM the same).

Fix this to do raw decoding so that we get better error messages in this case.