Request for review: 7154670: The instanceKlass _implementors[] and _nof_implementors are not needed for non-interface klass

[Jiangli Zhou]

Hi,

Please review following webrev that changes the 
instanceKlass::_implementors[] to an embedded instanceKlass field:

   http://cr.openjdk.java.net/~jiangli/7154670/webrev.00/

The embedded implementor field is only created for interfaces. Since the 
VM/compiler does not seem to use the implementor klassOop when there are 
more than one implementor and doesn't access _implementor[1], the 
embedded field is no longer a klassOop[2] array. It only holds one 
implementor klassOop now. When more than one implementor is loaded, the 
filed is set to the interface klassOop itself as a special indicator:

    Null: no implementor
    Implementor klassOop: one implementor
    Self: more than one implementor

The instanceKlass::_nof_implementors is removed as part of the change. 
For non-interface classes, this saves 12 bytes per class. For 
interfaces, it also saves 8 bytes per class.

Thanks,

Jiangli