Tests an issue where pattern matching matrices involving built-in
types was discarding default cases in some branches.

```ucm:hide
scratch/main> builtins.merge
```

```unison
f = cases
  0, 0 -> 0
  _, 1 -> 2
  1, _ -> 3
  _, _ -> 1

> f 0 0
> f 1 0
> f 0 1
> f 1 1
```