RFR (S): 8199567: [Nestmates] Cleanup instanceKlass.cpp

[David Holmes]

Bug: https://bugs.openjdk.java.net/browse/JDK-8199567
Webrev: http://cr.openjdk.java.net/~dholmes/8199567/webrev/

This is a simple "cleanup" previously suggested by John Rose for 
checking nest membership. Rather than checking for name equality and 
then checking the loaded class, we check if there's a loaded class first 
and if not fallback to the name check. This favours the case where the 
nestmate classes are already resolved and loaded.

There was no observable performance difference.

Testing: mach5 tiers 1-3 including nestmate tests

Thanks,
David