Method isLeaf

\brief Determine if a Varnode is a leaf within the CONCAT tree rooted at the given Varnode The CONCAT tree is the maximal set of Varnodes that are all inputs to CPUI_PIECE operations, with no other uses, and that all ultimately flow to the root Varnode. This method tests whether a Varnode is a leaf of this tree. \param rootVn is the given root of the CONCAT tree \param vn is the Varnode to test