Peek through barriers in Node::eqv_uncast()

[Roman Kennke]

>> Yes, indeed. But let's spin it a bit in Shenandoah first, shall we?
>> In-fact, I want to run the lucene suite a few more times before even
>> pushing it, each run takes 1-2hours so maybe until EOD? This frickin bug
>> tends to be very spurious.
> 
> Regardless if it fixes the current bug, it look as general thing to deal with for all GCs. So, why
> wait for those results? Maybe pass it via jdk-submit and RFR upstream directly?
> 
>> http://cr.openjdk.java.net/~rkennke/c2-eqv/webrev.01/
> 
> Looks good to me. The comment needs to mention GC barriers too?
> 
>  463   // Return whether two Nodes are equivalent, after stripping casting.
>  464   bool eqv_uncast(const Node* n) const;
> 
> Renaming the method would probably be quite intrusive...
> 

I added a comment and will push it as cherry-pick for:
https://bugs.openjdk.java.net/browse/JDK-8212603

Roman