Simplify and optimize the coloring algorithm.
     
    There's no need to first split the registers into constrained/unconstrained
    sets first and transform these sets into a list using an additional pass;
    instead we can directly place the registers on lists, where the constrained
    list is sorted by spill cost.
    Afterwards all that's left to do is to assign locations to the constrained
    and unconstrained registers by traversing the lists.
    
    
    
    git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13145 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02