Improve code generation for subtraction.
    
    Given integer constants x and n:
    1. If n + x does not overflow, then (x - c) + n is compiled
       to (x + n) - c.
    2. If n - x does not overflow, then (c - x) + n is compiled
       to c + (n - x).
    
    git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14266 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02