[js] switch on const value leaves behind empty switch

Target JS, with analyzer. Same behavior on 3.4.7 and on Haxe 4 nightly as of 2019/02/13.
The following code works as expected, but leaves behind an empty switch. See http://try-haxe.mrcdk.com/#Df7e7.
```haxe
class Test {
static function main() {
// var anim = '';
var state = GoingLeft;
inline function set(s) {
state = s;
// anim = getAnim(s);
trace(getAnim(s));
}
if(Math.random() > 0.5) set(GoingLeft);
// trace(anim);
trace(state);
}

static inline function getAnim(state:ActorStateType):String {
return switch(state) {
case GoingLeft: 'left';
case GoingRight: 'right';
}
}
}

@:enum abstract ActorStateType(Int) from Int to Int {
var GoingLeft = 0;
var GoingRight = 1;
}
```
This code is in the compiled js:
```js
switch(s) {
case 0:
break;
case 1:
break;
}
```
Furthermore, uncommenting the extra lines also produces:
```js
switch(s) {
case 0:
anim = "left";
break;
case 1:
break;
}
```