[arm] Fix bug in div/mod by constant.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14265 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
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