Unions with 4 or more cases silently fail to utilize UseNullAsTrueValue.

Repro. case in F# 4.4.0.0 -

```
[]
type T = A | B of int | C of int | D of int;;
A;;
```

In F# interactive, here you'll see that A is a proper value, which is not desired.

```
[]
type T = A | B of int | C of int;;
A;;
```

In F# interactive, here you'll see that A is a null, which is desired.

Would be a nice optimization to have this work for 4 cases as here - https://github.com/bryanedds/Prime/blob/master/Prime/Prime/Prime/Vmap.fs#L25-L30

NOTE: Issue moved from here - https://github.com/fsharp/fsharp/issues/510

Cheers!