Incorrect activity guessed for ForwardMode

```
using Reactant, Enzyme

function f(x)
    return sum(abs2, x; dims=(2, 3))
end

x = Reactant.to_rarray(rand(Float32, 3, 4, 5, 6))
bx = Reactant.to_rarray(rand(Float32, 3, 4, 5, 6))

@code_hlo Enzyme.autodiff(Forward, f, Duplicated(x, bx))
```

`A` inside `overloaded_autodiff` is `Const{Union{}}`.