[reanalyze] [bug] division_by_zero exception analysis should check constant denominators

[tx46]

repro:

let foo = (x: int) => {
  x / 10
}

reanalyze gives:

{
  "name": "Exception Analysis",
  "kind": "warning",
  "file": "Foo.res",
  "range": [20,4,20,7],
  "message": "foo might raise Division_by_zero (Foo.res:22:4) and is not annotated with @raises(Division_by_zero)",
  "annotate": { "line": 20, "character": 0, "text": "@raises(Division_by_zero)\n", "action": "Add @raises annotation"}
}

it's trivially clear that division by zero can't happen, and so reanalyze should not suggest the annotation.