RFR (S) 8215575: C2 crash: assert(get_instanceKlass()->is_loaded()) failed: must be at least loaded

[coleen.phillimore at oracle.com]

Summary: Set InstanceKlass::loaded before adding classes to the subklass 
list, which can be read concurrently by the compiler.

Thanks to Erik for the diagnosis and suggested fix.  See bug comments 
for more details.

Tested with hs-tier1-3, 6 and 8.

open webrev at http://cr.openjdk.java.net/~coleenp/8215575.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8215575

Thanks,
Coleen