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

1 files changed, 2 insertions, 2 deletions

diff --git a/compiler/GHC/Core/Opt/SetLevels.hs b/compiler/GHC/Core/Opt/SetLevels.hs
index 0a4d4541f4..00d38f40cd 100644
--- a/compiler/GHC/Core/Opt/SetLevels.hs
+++ b/compiler/GHC/Core/Opt/SetLevels.hs
@@ -103,7 +103,7 @@ import GHC.Types.Unique.Set   ( nonDetStrictFoldUniqSet )
 import GHC.Types.Unique.DSet  ( getUniqDSet )
 import GHC.Types.Var.Env
 import GHC.Types.Literal      ( litIsTrivial )
-import GHC.Types.Demand       ( StrictSig, Demand, isStrictDmd, splitStrictSig, prependArgsStrictSig )
+import GHC.Types.Demand       ( StrictSig, Demand, isStrUsedDmd, splitStrictSig, prependArgsStrictSig )
 import GHC.Types.Cpr          ( mkCprSig, botCpr )
 import GHC.Types.Name         ( getOccName, mkSystemVarName )
 import GHC.Types.Name.Occurrence ( occNameString )
@@ -469,7 +469,7 @@ lvlApp env orig_expr ((_,AnnVar fn), args)
     lvl_arg :: [Demand] -> CoreExprWithFVs -> LvlM ([Demand], LevelledExpr)
     lvl_arg strs arg | (str1 : strs') <- strs
                      , is_val_arg arg
-                     = do { arg' <- lvlMFE env (isStrictDmd str1) arg
+                     = do { arg' <- lvlMFE env (isStrUsedDmd str1) arg
                           ; return (strs', arg') }
                      | otherwise
                      = do { arg' <- lvlMFE env False arg