RFR: Cherry-pick: 8213746: GC/C2 abstraction for C2 matcher

[Aleksey Shipilev]

On 11/21/18 9:18 PM, Roman Kennke wrote:
> In order to ease merge from upstream, I'd like to cherry-pick "8213746:
> GC/C2 abstraction for C2 matcher" plus the corresponding Shenandoah changes:
> 
> http://cr.openjdk.java.net/~rkennke/JDK-8213746-shenandoah/webrev/

*) Whitespace oddity at L1445:

1444       Node* newval = n->in(MemNode::ValueIn);
1445       Node* oldval  = n->in(LoadStoreConditionalNode::ExpectedIn);

Otherwise good.

-Aleksey