RFR(S): barrier moved due to null checks needs to always fix memory edges

[Roland Westrelin]

>  *) This line is commented, and can be removed?
>
>   1973   //ResourceMark rm; // register_new_node makes an array grow

It's a reminder that we shouldn't put a ResourceMark here but you're
right no need to keep it forever. I'll remove it.

Roland.