RFR: Load reference barriers

[Roman Kennke]

Will do.

FWIW, I tried removing those lines and hotspot_gc_shenandoah seemed happy.

I consider the last webrev approved and will push later tonight.

Thanks,
Roman

>> 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.
>