8231630: Optimize boot loader with no bootclasspath append entry. Summary: Optimize SystemDictionary::load_instance_class for boot loader. Immediately return NULL if the requested class is either in the unnamed package or unnamed module, or in a module not defined to the boot loader, when there is no bootclasspath append entry. Reviewed-by: martin, aeubanks, dthomson, redestad, lfoltan, iklam

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