Demand: Interleave usage and strictness demands (#18903)

As outlined in #18903, interleaving usage and strictness demands not
only means a more compact demand representation, but also allows us to
express demands that we weren't easily able to express before.

Call demands are *relative* in the sense that a call demand `Cn(cd)`
on `g` says "`g` is called `n` times. *Whenever `g` is called*, the
result is used according to `cd`". Example from #18903:

```hs
h :: Int -> Int
h m =
  let g :: Int -> (Int,Int)
      g 1 = (m, 0)
      g n = (2 * n, 2 `div` n)
      {-# NOINLINE g #-}
  in case m of
    1 -> 0
    2 -> snd (g m)
    _ -> uncurry (+) (g m)
```

Without the interleaved representation, we would just get `L` for the
strictness demand on `g`. Now we are able to express that whenever
`g` is called, its second component is used strictly in denoting `g`
by `1C1(P(1P(U),SP(U)))`. This would allow Nested CPR to unbox the
division, for example.

Fixes #18903.
While fixing regressions, I also discovered and fixed #18957.

Metric Decrease:
    T13253-spj

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