RFR (T) 8242898: Clean up InstanceKlass::_array_klasses

[coleen.phillimore at oracle.com]

Summary: Use more specific ObjArrayKlass type.

This avoids some casts.  Tested with tier1-3.

open webrev at http://cr.openjdk.java.net/~coleenp/2020/8242898.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8242898

thanks,
Coleen