Review request: 8000459: assert(java_lang_String::is_instance(entry)) failure with various mlvm tests

[Jiangli Zhou]

Please review following fix for 8000459:

   http://cr.openjdk.java.net/~jiangli/8000459/webrev/

With JSR292, a constant pool String entry could be patched to a 
non-string oop. The assert in VM_HeapWalkOperation::iterate_over_class() 
needs to be adjusted to reflect that.

Thanks,

Jiangli