[OM] Missing verifier that output field names don't collide with inputs?

Consider the following:

``` mlir
om.class @Bar_Class(
  %a: !om.integer
) -> (
  a: !om.integer
) {
  om.class.fields %a : !om.integer
}
```

The input and output have the same name `a`. However, there is no verifier to check this. Is this a bug?

I ran into this when using the evaluator where I accidentally did this and some of the logic for getting the fields was then getting the wrong field.