Method alloc_varnode

src/core/include/megbrain/graph/cg.h:254–256

     */
    template <typename... Args>
    VarNode* alloc_varnode(Args&&... args) {
        return new (alloc_varnode_storage()) VarNode(std::forward<Args>(args)...);
    }

    inline void free_varnode(VarNode* var) {
        var->~VarNode();