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