RFR: Extract Shenandoah superword check into ShenandoahBarrierSetC2

[Roman Kennke]

> 
>> Let me take another look at this one. I have to figure out why it's
>> needed again. I thought some more about whether this could be upstreamed
>> even though it's shenandoah and I may give it a try.
> 
> I ran a bunch of tests without that change and nothing broke. Let's
> remove it. We can add it back if needed:
> 
> http://cr.openjdk.java.net/~roland/shenandoah/undo-superword/webrev.00/
> 
> Roland.

Ok.