Register `id_stmt` with its base identity, if any.

def register_identity(self, id_stmt):
        """Register `id_stmt` with its base identity, if any.
        """
        bst = id_stmt.search_one("base")
        if bst:
            bder = self.identity_deps.setdefault(bst.i_identity, [])
            bder.append(id_stmt)