RFR: Simpler obj equals barrier for C2

[Roman Kennke]

>> http://cr.openjdk.java.net/~rkennke/c2-obj-eq-barrier/webrev.01/
> 
> I doubt the changes in library_call.cpp are ok. WB produces memory. So
> dropping the mem phis and moving the WBs at a different location is
> likely to break something.
> 
> Roland.

Ok. But how would I notice? The test suite works ok. I'll also run some
other stuff (e.g. specjvm/jbb).

Also, the WB code only produces memory in the Shenandoah brooks pointer
slice(s), and it takes care of producing the memphis as needed. Maybe
this is good enough?

Roman