RFR: rework CAS support

[Roman Kennke]

> 
> http://cr.openjdk.java.net/~roland/shenandoah/cas-gcapi/webrev.00/
> 
> With this change, CAS is implemented with its own set of C2 nodes and
> its own c1 LIR_Op and creation of the nodes/LIR_Op is done behind the GC
> api.
> 
> Roland.
> 

Excellent! This should reduce our upstream diff considerably. And the
new stuff to support new C2 nodes should be ok too. Patch looks good,
please push if it passes tier3_gc_shenandoah.

Thanks,
Roman