objArrayOopDesc::atomic_compare_exchange_oop() must use obj+offset in HeapAccess call

objArrayOopDesc::atomic_compare_exchange_oop() currently computes and uses a raw pointer to pass to HeapAccess::oop_atomic_cmpxchg(). It should use the object+offset variant because it's accessing the heap and GC might need to resolve the target object (e.g. Shenandoah).