exprIsConApp_maybe leads to multiple simplifier iterations

Suppose we have a simple data constructor wrapper like this (this one might have come from a data family instance):

$WK x y = K x y |> co

Now suppose the simplifier sees

    case ($WK e1 e2) |> co2 of
       K p q ->  case q of ...

exprIsConApp_maybe expands the wrapper on the fly (see Note [beta-reduction in exprIsConApp_maybe]). It effectively expands that ($WK e1 e2) to let x = e1; y = e2 in K x y |> co. So the Simplifier ends up producing this:

    let x = e1; y = e2
    in case x of ...

But suppose q was used just once in the body of the K p q alternative; we don't want to wait a whole Simplifier iteration to inline that x. (e1 might be another constructor for example.)

The error is that in exprIsConApp_maybe we are creating a let for every (non-trivial) argument. It would be easy instead to take advantage of the occurrence-info on x and y in the unfolding of $WK. Since in $WK both x and y occur once, we want to effectively expand ($WK e1 e2) to K e1 e2 |> co. Easy really. A one line change. Take much longer to describe than to implement!