Tech debt: IfNull rewrite is in wrong module

[nj1973]

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.