core.simd ubyte16 initialization weirdness.

[Richard (Rikki) Andrew Cattermole]

Don't forget to type bad2 which gives the same result as the good one. 
Otherwise it only has 7 elements in it.

```d
static immutable ubyte16 bad2 = [1, 2, 3, 4, 5, 6, 7];
```

```
[1, 2, 3, 4, 5, 6, 7, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[101, 102, 103, 104, 105, 106, 107, 100, 100, 100, 100, 100, 100, 100, 
100, 100]
```