Nested CPR light (#19398)

While fixing #19232, it became increasingly clear that the vestigial
hack described in `Note [Optimistic field binder CPR]` is complicated
and causes reboxing. Rather than make the hack worse, this patch
gets rid of it completely in favor of giving deeply unboxed parameters
the Nested CPR property. Example:
```hs
f :: (Int, Int) -> Int
f p = case p of
 (x, y) | x == y    = x
        | otherwise = y
```
Based on `p`'s `idDemandInfo` `1P(1P(L),1P(L))`, we can see that both
fields of `p` will be available unboxed. As a result, we give `p` the
nested CPR property `1(1,1)`. When analysing the `case`, the field
CPRs are transferred to the binders `x` and `y`, respectively, so that
we ultimately give `f` the CPR property.

I took the liberty to do a bit of refactoring:

- I renamed `CprResult` ("Constructed product result result") to plain
  `Cpr`.
- I Introduced `FlatConCpr` in addition to (now nested) `ConCpr` and
  and according pattern synonym that rewrites flat `ConCpr` to
  `FlatConCpr`s, purely for compiler perf reasons.
- Similarly for performance reasons, we now store binders with a
  Top signature in a separate `IntSet`,
  see `Note [Efficient Top sigs in SigEnv]`.
- I moved a bit of stuff around in `GHC.Core.Opt.WorkWrap.Utils` and
  introduced `UnboxingDecision` to replace the `Maybe DataConPatContext`
  type we used to return from `wantToUnbox`.
- Since the `Outputable Cpr` instance changed anyway, I removed the
  leading `m` which we used to emit for `ConCpr`. It's just noise,
  especially now that we may output nested CPRs.

Fixes #19398.

1 files changed, 1 insertions, 1 deletions

diff --git a/testsuite/tests/deSugar/should_compile/T2431.stderr b/testsuite/tests/deSugar/should_compile/T2431.stderr
index 69f40310b4..8b3f8a53b6 100644
--- a/testsuite/tests/deSugar/should_compile/T2431.stderr
+++ b/testsuite/tests/deSugar/should_compile/T2431.stderr
@@ -7,7 +7,7 @@ Result size of Tidy Core
 T2431.$WRefl [InlPrag=INLINE[final] CONLIKE] :: forall a. a :~: a
 [GblId[DataConWrapper],
  Caf=NoCafRefs,
- Cpr=m1,
+ Cpr=1,
  Unf=Unf{Src=InlineStable, TopLvl=True, Value=True, ConLike=True,
          WorkFree=True, Expandable=True,
          Guidance=ALWAYS_IF(arity=0,unsat_ok=True,boring_ok=False)