Less side effects in InstanceRefKlass::trace_reference_gc

The InstanceRefKlass::trace_reference_gc function is called when using oop_iterate on a reference object. Since oop_iterate is a general iterator that can be used for anything, including verification code, it would be desirable for this code to not have subtle side effects. We currently apply load barriers which "fix" pointers in the heap. As a result, verification code that was meant to detect bugs, can end up fixing and hiding the bug. This CR aims to address this.