Tech debt: IfNull rewrite is in wrong module

I noticed that the lines below in `third_party/ibis/ibis_impala/api.py` effect more than just Impala:

```
@rewrites(ops.IfNull)
def _if_null(op):
return ops.Coalesce((op.arg, op.ifnull_expr))
```

If I comment out the lines then MySQL generated SQL changes too.

We need to review whether we should move this code to `operations.py` or remove it and add engine specific rewrites where they are needed.