RFR: Fix CAS-obj with expected=NULL

[Roland Westrelin]

> http://cr.openjdk.java.net/~rkennke/fix-cas-obj-null/webrev.00/

That's not right. Matching introduces Binary nodes:

      case Op_CompareAndSwapN: {   // Convert trinary to binary-tree
        Node *newval = n->in(MemNode::ValueIn );
        Node *oldval  = n->in(LoadStoreConditionalNode::ExpectedIn);
        Node *pair = new BinaryNode( oldval, newval );
        n->set_req(MemNode::ValueIn,pair);
        n->del_req(LoadStoreConditionalNode::ExpectedIn);

so index of inputs changes and existing code is good.

Roland.