RFR 8139163: InstanceKlass::cast passes through NULL

[Coleen Phillimore]

Summary: Reduce raw (InstanceKlass*) casts and InstanceKlass::cast, 
which no long allows null

Somewhat of a tedious thing to code review (sorry).   Tested with 
internal remote-build-and-test.

open webrev at http://cr.openjdk.java.net/~coleenp/8139163.01/
bug link https://bugs.openjdk.java.net/browse/JDK-8139163.01

Thanks,
Coleen