Re-engineer the binder-swap transformation

The binder-swap transformation is implemented by the occurrence
analyser -- see Note [Binder swap] in OccurAnal. However it had
a very nasty corner in it, for the case where the case scrutinee
was a GlobalId.  This led to trouble and hacks, and ultimately
to #16296.

This patch re-engineers how the occurrence analyser implements
the binder-swap, by actually carrying out a substitution rather
than by adding a let-binding.  It's all described in
Note [The binder-swap substitution].

I did a few other things along the way

* Fix a bug in StgCse, which could allow a loop breaker to be CSE'd
  away.  See Note [Care with loop breakers] in StgCse.  I think it can
  only show up if occurrence analyser sets up bad loop breakers, but
  still.

* Better commenting in SimplUtils.prepareAlts

* A little refactoring in CoreUnfold; nothing significant
  e.g. rename CoreUnfold.mkTopUnfolding to mkFinalUnfolding

* Renamed CoreSyn.isFragileUnfolding to hasCoreUnfolding

* Move mkRuleInfo to CoreFVs

We observed respectively 4.6% and 5.9% allocation decreases for the following
tests:

Metric Decrease:
    T9961
    haddock.base

1 files changed, 2 insertions, 5 deletions

diff --git a/compiler/GHC/IfaceToCore.hs b/compiler/GHC/IfaceToCore.hs
index 0024d92037..559587664e 100644
--- a/compiler/GHC/IfaceToCore.hs
+++ b/compiler/GHC/IfaceToCore.hs
@@ -63,7 +63,6 @@ import GHC.Types.Name
 import GHC.Types.Name.Env
 import GHC.Types.Name.Set
 import GHC.Core.Op.OccurAnal ( occurAnalyseExpr )
-import GHC.Types.Demand
 import GHC.Types.Module
 import GHC.Types.Unique.FM
 import GHC.Types.Unique.Supply
@@ -1506,14 +1505,12 @@ tcUnfolding toplvl name _ info (IfCoreUnfold stable if_expr)
                       | otherwise = InlineRhs
         ; return $ case mb_expr of
             Nothing -> NoUnfolding
-            Just expr -> mkUnfolding dflags unf_src
-                           True {- Top level -}
-                           (isBottomingSig strict_sig)
-                           expr
+            Just expr -> mkFinalUnfolding dflags unf_src strict_sig expr
         }
   where
      -- Strictness should occur before unfolding!
     strict_sig = strictnessInfo info
+
 tcUnfolding toplvl name _ _ (IfCompulsory if_expr)
   = do  { mb_expr <- tcPragExpr True toplvl name if_expr
         ; return (case mb_expr of