8029858: Enhance array copies

Summary: Just read the source element once
Reviewed-by: coleenp, ahgross

src/share/vm/oops/objArrayKlass.cpp

@@ -269,7 +269,7 @@ template <class T> void ObjArrayKlass::do_copy(arrayOop s, T* src,
         if (element_is_null ||
             (new_val->klass())->is_subtype_of(bound)) {
           bs->write_ref_field_pre(p, new_val);
-          *p = *from;
+          *p = element;
         } else {
           // We must do a barrier to cover the partial copy.
           const size_t pd = pointer_delta(p, dst, (size_t)heapOopSize);