CountedLoopEndNode::phi() should only return PhiNodes

CountedLoopEndNode::phi() currently does not check if the node is a PhiNode and may therefore return other nodes (see JDK-8134739). We should change the implementation to always either return a PhiNode or NULL (use tmp->in(1)->as_Phi()) and adapt the call sites accordingly. For example, PhiNode::Value() currently expects phi() to return "something" even if the loop does not contain a PhiNode.