Only apply forin check to for…in loops.

This change corrects the behaviour of JSHint’s forin option
on for…of loops; the two share many checks, but this:

    for (var x of [1, 2, 3]) {
        // …
    }

should not result in the warning reported by jshint/jshint#1079:

> The body of a for in should be wrapped in an if statement to filter
> unwanted properties from the prototype.