[interf] Don't record "ill-typed" interferences.
    
    The coloring algorithm uses only the "well-typed" interferences (registers
    with same class). This is because we don't assign the same stack slot to
    registers of different register class.
    
    git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13207 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02