I found that a tricky thing can happen when constant inference derives
`Const`-result while non-constant inference has derived (non-constant)
`InterConditional` result beforehand. In such a case, currently we discard
the result with constant infernece (since `!(Const ⊑ InterConditional)`),
but we can achieve more accuracy by not discarding that `Const`-information, e.g.:
```julia
julia> iszero_simple(x) = x === 0
iszero_simple (generic function with 1 method)

julia> @test Base.return_types() do
           iszero_simple(0) ? nothing : missing
       end |> only === Nothing
Test Passed
```

Co-authored-by: Kristoffer Carlsson <kcarlsson89@gmail.com>