Combine the first two coloring passes. 
     
    This combines the preallocation of spilled registers and splitting of
    the remaining registers into constrained and unconstrained, and thereby
    saves one pass on the register set.
    
    
    
    git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13143 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02