RFR (S): 8198561: Make oop and narrowOop always have their own class type

[Erik Österlund]

Hi,

Making oop sometimes map to class types and sometimes to primitives 
comes with some unfortunate problems. Advantages of making them always 
have their own type include:

1) Not getting compilation errors in configuration X but not Y
2) Making it easier to adopt existing code to use Shenandoah equals 
barriers
3) Recognize oops and narrowOops safely in template

Therefore, I would like to make both oop and narrowOop always map to a 
class type consistently.

Webrev:
http://cr.openjdk.java.net/~eosterlund/8198561/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8198561

Thanks,
/Erik