Fix registerizeHarder handling unlabelled continue

The registerizeHarder pass was not correctly handling an unlablled 'continue' inside a 'switch'.  Given e.g. this:

```
while(1) {
  switch (n) {
    case 0:
      continue
    ...
  }
}
```

It would treat the `continue` as if it jumped to the start of the `switch` context (which is nonsense and caused an error) rather than the start of the enclosing `while` context.  This PR fixes it by ensuring that the `onContinue` junction for a `switch` is set to the value from its enclosing context.

This fixes `asm3.test_freetype` for me.