Only add extra loop for internal clocks generated by delayed assignment.

src:
	* V3Order.cpp (OrderVisitor::visit(AstNodeVarRef *): Mark post
	generated vars as delayed.
	(OrderVisitor::processCircular): Only mark circular internal
	clocks generated by delayed assignment.
	* V3OrderGraph (OrderVarVertex::m_isDelayed): Created.
	(OrderVarVertex::OrderVarVertex): Initialize m_isDelayed.
	(isDelayed(bool), isDelayed): Created.