Demand: Nested strict product demands (#18885)wip/T18885

Fixing #18903 gives us enough expressiveness to tackle #18885, where we
have

```hs
f :: Int -> Int
f y =
  let x
       | expensive y == 1 = (expensive (y+1), expensive (y+2))
       | otherwise        = (expensive (y+3), expensive (y+4))
  in case () of
       _ | expensive (y+5) == 42 -> fst x
       _ | expensive (y+6) == 41 -> fst x + snd x
       _ | otherwise             -> 0
```

Here, we used to give `x` demand `1P(1P(U),1P(U))`. The outer `1`
is because `x` is used lazily and the inner `1`s are redundant with
that fact. That leaves some expressiveness on the table. After this
change, we infer `1P(SP(U),1P(U))`, meaning that *whenever we evaluate
`x`*, we evaluate its first component strictly, effectively making
strictness product demands apply *relatively*. Usage product demands
still apply absolutely, though.

More details on how we could exploit the new language in
`Note [Absent sub-demands]`.

Fixes #18885.

There's a single remaining regression in `T9630`, which increases +16%
in residency but decreases slightly in total allocations. I checked
the heap profile, which doesn't suggest any obvious regressions.
Ticky doesn't point to the reason either, because total allocations
actually improved. I think it's OK to just accept it.

Metric Increase:
    T9630

1 files changed, 7 insertions, 5 deletions

diff --git a/compiler/GHC/Core/Opt/DmdAnal.hs b/compiler/GHC/Core/Opt/DmdAnal.hs
index 4869fb1fa9..ae8aab18a8 100644
--- a/compiler/GHC/Core/Opt/DmdAnal.hs
+++ b/compiler/GHC/Core/Opt/DmdAnal.hs
@@ -233,9 +233,10 @@ dmdAnal' env dmd (Case scrut case_bndr ty [(alt, bndrs, rhs)])
         (rhs_ty, rhs')           = dmdAnal env dmd rhs
         (alt_ty1, dmds)          = findBndrsDmds env rhs_ty bndrs
         (alt_ty2, case_bndr_dmd) = findBndrDmd env False alt_ty1 case_bndr
-        -- Evaluation cardinality on the case binder is irrelevant and a no-op.
-        -- What matters is its nested sub-demand!
-        (_ :* case_bndr_sd)      = case_bndr_dmd
+        -- The peelDmd below will lazify the relative sub-demands if the
+        -- case_bndr_dmd had lazy evaluation cardinality.
+        -- See Note [Absent sub-demand] in GHC.Types.Demand
+        case_bndr_sd             = peelDmd case_bndr_dmd
         -- Compute demand on the scrutinee
         (bndrs', scrut_sd)
           | DataAlt _ <- alt
@@ -388,9 +389,10 @@ dmdAnalSumAlt :: AnalEnv -> SubDemand -> Id -> Alt Var -> (DmdType, Alt Var)
 dmdAnalSumAlt env dmd case_bndr (con,bndrs,rhs)
   | (rhs_ty, rhs') <- dmdAnal env dmd rhs
   , (alt_ty, dmds) <- findBndrsDmds env rhs_ty bndrs
-  , let (_ :* case_bndr_sd) = findIdDemand alt_ty case_bndr
+  , let case_bndr_dmd = findIdDemand alt_ty case_bndr
         -- See Note [Demand on scrutinee of a product case]
-        id_dmds             = addCaseBndrDmd case_bndr_sd dmds
+        -- See Note [Absent sub-demand] in GHC.Types.Demand
+        id_dmds       = addCaseBndrDmd (peelDmd case_bndr_dmd) dmds
   = (alt_ty, (con, setBndrsDemandInfo bndrs id_dmds, rhs'))
 
 {-