[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.

Start at the OM class verifier and the evaluator path that resolves fields, using the reproducer to trace how identical input and output names are handled. Done means the invalid name collision is rejected by verification and the evaluator no longer receives an ambiguous field definition.

Written by the indexing model from the issue text.