CprAnal: Activate Sum CPR for local bindings

We've had Sum CPR (#5075) for top-level bindings for a couple of years now.
That begs the question why we didn't also activate it for local bindings, and
the reasons for that are described in `Note [CPR for sum types]`. Only that it
didn't make sense! The Note said that Sum CPR would destroy let-no-escapes, but
that should be a non-issue since we have syntactic join points in Core now and
we don't WW for them (`Note [Don't w/w join points for CPR]`).

So I simply activated CPR for all bindings of sum type, thus fixing #5075 and
\#16570. NoFib approves:

```
--------------------------------------------------------------------------------
        Program         Allocs    Instrs
--------------------------------------------------------------------------------
  comp_lab_zift          -0.0%     +0.7%
          fluid          +1.7%     +0.7%
        reptile          +0.1%     +0.1%
--------------------------------------------------------------------------------
            Min          -0.0%     -0.2%
            Max          +1.7%     +0.7%
 Geometric Mean          +0.0%     +0.0%
```

There were quite a few metric decreases on the order of 1-4%, but T6048 seems to
regress significantly, by 26.1%. WW'ing for a `Just` constructor and the nested
data type meant additional Simplifier iterations and a 30% increase in term
sizes as well as a 200-300% in type sizes due to unboxed 9-tuples. There's not
much we can do about it, I'm afraid: We're just doing much more work there.

Metric Decrease:
    T12425
    T18698a
    T18698b
    T20049
    T9020
    WWRec
Metric Increase:
    T6048

2 files changed, 36 insertions, 10 deletions

diff --git a/testsuite/tests/stranal/sigs/T5075.hs b/testsuite/tests/stranal/sigs/T5075.hs
index c35409aa67..15b1357446 100644
--- a/testsuite/tests/stranal/sigs/T5075.hs
+++ b/testsuite/tests/stranal/sigs/T5075.hs
@@ -1,11 +1,31 @@
--- | This module currently asserts that we trim CPR for local bindings
--- returning a sum. We can hopefully give @loop@ a CPR signature some day, but
--- we first have to fix #5075/#16570.
+-- | This module currently asserts that we give functions that always return
+-- the same constructor of a sum type the CPR property.
 module T5075 where
 
 -- Omission of the type signature is deliberate, otherwise we won't get a join
 -- point (this is up to the desugarer, not sure why).
--- loop :: (Ord a, Num a) => a -> Either a b
-loop x = case x < 10 of
-  True -> Left x
-  False -> loop (x*2)
+-- f :: (Ord a, Num a) => a -> Either a b
+f x = case x < 10 of
+  True  -> Left x
+  False -> f (x*2)
+
+-- Similarly a join point. Should WW nonetheless
+g :: Int -> Int -> Maybe Int
+g x y = go x
+  where
+    go x = case x < y of
+      True  -> Just x
+      False -> go (x*2)
+
+-- Here, go is not a join point, but still should be WW'd for Just.
+-- Unfortunately, CPR can't see that (+?) returns Just, so h won't get the CPR
+-- property. It probably could by only considering the @Just@ case of the
+-- inlined (+?).
+h :: Int -> Maybe Int
+h x = go x +? go (x+1)
+  where
+    Just x +? Just y = Just (x + y)
+    _      +? _      = Nothing
+    go z
+      | z > 10    = Just (x + z)
+      | otherwise = go (z*2)
diff --git a/testsuite/tests/stranal/sigs/T5075.stderr b/testsuite/tests/stranal/sigs/T5075.stderr
index e17d5e7c5c..c9625db721 100644
--- a/testsuite/tests/stranal/sigs/T5075.stderr
+++ b/testsuite/tests/stranal/sigs/T5075.stderr
@@ -1,18 +1,24 @@
 
 ==================== Strictness signatures ====================
 T5075.$trModule:
-T5075.loop: <SP(A,A,SCS(C1(L)),A,A,A,A,A)><LP(A,A,LCL(C1(L)),A,A,A,L)><L>
+T5075.f: <SP(A,A,SCS(C1(L)),A,A,A,A,A)><LP(A,A,LCL(C1(L)),A,A,A,L)><L>
+T5075.g: <1P(L)><SP(L)>
+T5075.h: <SP(L)>
 
 
 
 ==================== Cpr signatures ====================
 T5075.$trModule:
-T5075.loop:
+T5075.f: 1
+T5075.g: 2(1)
+T5075.h:
 
 
 
 ==================== Strictness signatures ====================
 T5075.$trModule:
-T5075.loop: <1P(A,A,SCS(C1(L)),A,A,A,A,A)><LP(A,A,LCL(C1(L)),A,A,A,L)><L>
+T5075.f: <1P(A,A,SCS(C1(L)),A,A,A,A,A)><LP(A,A,LCL(C1(L)),A,A,A,L)><L>
+T5075.g: <1P(L)><SP(L)>
+T5075.h: <1P(L)>