RFR: Load reference barriers

[Roland Westrelin]

> Do you think it's still useful to have this code:
>   Node* base = find_bottom_mem(ctrl, phase);
>   MergeMemNode* mm = MergeMemNode::make(base);
>   mm->set_memory_at(Compile::AliasIdxRaw, raw_mem);
>
> or would mm already contain the raw_mem?

I think it should be left as it is.

Roland.