this code
```
inductive N where
| cons : (Nat -> N) -> N
mutual
def f : N -> Nat
| .cons a => g (a 32) + 1
termination_by structural n => n
def g : N -> Nat
| .cons a => f (a 42) + 1
termination_by structural n => n
end
```
would break. When searching for the right `belowDict` we now have to,
evne after instantiating the paramters for a reflexive argument, again
search through a bunch of `PProd`s.
(Instead of searching we could pass down the index, but since we are
searching anyways in this function let's just re-use.)
Fixes: #4726
