hxcpp 3.2 regression: re-assignment of a dynamic variable to a different type fails

Here is a sample code:

```
class Foo
{
public static function main()
{
var json = haxe.Json.parse("{\"array\":[\"Hello\"]}");

var field = Reflect.field(json, "array");

if (field == null) {
throw("Reflect failed!");
}

if (Std.is(field, Array)) {
var array : Array = field;
if (0 < array.length) {
trace("Assigning field from [" + field +
"] to [" + array[0] + "]");
field = array[0];
}
}

trace("field is " + field);
}
}
```

The variable 'field' is re-used both as an Array, and as a value within the Array. But since the type of 'field' is Dynamic (as inferred by the return type of Reflect.field()), assigning it to any value subsequently should work.

Expected result:

Foo.hx:17: Assigning field from [[Hello]] to [Hello]

Foo.hx:23: field is Hello

Actual results:
- haxe 3.1.3, neko target: as expected
- haxe 3.1.3, hxcpp target: as expected
- haxe 3.2, neko target: as expected
- haxe 3.2, hxcpp target: invalid result:

  Foo.hx:17: Assigning field from [[Hello]] to [Hello]

  Foo.hx:23: field is null