RFR (XS): 80357290: Code using assert(is_oop_or_null) needs better error messages

[Marcus Larsson]

Hi,

Can I have reviews for this small change adding oop information to 
is_oop_or_null assert fail messages?

CR:
http://cr.openjdk.java.net/~brutisso/webrev-8035729/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8035729/

Thanks,
Marcus