Enum operations should produce base type

Enum values are not strongly typed, so final switch is not compile-time exhaustive.

```d
enum E
{
	a = 2,
	b = 3
}
unittest
{
	E e = E.b;
	e++;
	e *= 2;
	//core.exception.SwitchError@enum.d(13): No appropriate switch clause found
	final switch (e)
	{
		case E.a, E.b:
	}
	e = E.b ^ E.a;
	assert(e == 1);
	assert(e >= E.min); //fails
}
```