RFR: Clean up changes in phaseX.cpp

[Roman Kennke]

Ok. I pushed only the phaseX.cpp changes.

I've also filed
https://bugs.openjdk.java.net/browse/JDK-8214057

to cover has_special_unique_user().

Thanks,
Roman

>> This cleans up our changes around has_special_unique_user(). Looks like
>> all uses of this method are followed by calls into BarrierSetC2, which
>> we can use to achieve what we need.
> 
> The phaseX.cpp is useless so I'm fine with it being removed. I disagree
> with the other changes. Sure it might be functionally equivalent, but
> Node::has_special_unique_user() has a purpose which is to flag nodes
> that require special work when they have a special unique user. That's
> the case of our write barriers. Shuffling the code around to hide our
> code being the gc api makes the code less readable and more error prone.
> 
> Roland
>