Note [No nested Unboxed inside Boxed in demand signature]

Consider
```
f p@(x,y)
  | even (x+y) = []
  | otherwise  = [p]
```
Demand analysis will infer that the function body puts a demand of `1P(1!L,1!L)`
on 'p', e.g., Boxed on the outside but Unboxed on the inside. But worker/wrapper
can't unbox the pair components without unboxing the pair! So we better say
`1P(1L,1L)` in the demand signature in order not to spread wrong Boxity info.
That happens via the call to trimBoxity in 'finaliseArgBoxities'/'finaliseLetBoxity'.