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

3 files changed, 19 insertions, 35 deletions

diff --git a/testsuite/tests/simplCore/should_compile/T17901.stdout b/testsuite/tests/simplCore/should_compile/T17901.stdout
index 406e81ef5f..99969cc0c1 100644
--- a/testsuite/tests/simplCore/should_compile/T17901.stdout
+++ b/testsuite/tests/simplCore/should_compile/T17901.stdout
@@ -4,13 +4,11 @@
                    C -> wombat1 T17901.C
   = \ (@p) (wombat1 :: T -> p) (x :: T) ->
       case x of wild { __DEFAULT -> wombat1 wild }
-                 (wombat2 [Occ=Once*!] :: S -> p)
-                   SA _ [Occ=Dead] -> wombat2 wild;
-                   SB -> wombat2 T17901.SB
+         Tmpl= \ (@p) (wombat2 [Occ=Once!] :: S -> p) (x [Occ=Once] :: S) ->
+                 case x of wild [Occ=Once] { __DEFAULT -> wombat2 wild }}]
   = \ (@p) (wombat2 :: S -> p) (x :: S) ->
       case x of wild { __DEFAULT -> wombat2 wild }
-                 (wombat3 [Occ=Once*!] :: W -> p)
-                   WB -> wombat3 T17901.WB;
-                   WA _ [Occ=Dead] -> wombat3 wild
+         Tmpl= \ (@p) (wombat3 [Occ=Once!] :: W -> p) (x [Occ=Once] :: W) ->
+                 case x of wild [Occ=Once] { __DEFAULT -> wombat3 wild }}]
   = \ (@p) (wombat3 :: W -> p) (x :: W) ->
       case x of wild { __DEFAULT -> wombat3 wild }
diff --git a/testsuite/tests/simplCore/should_compile/T7360.hs b/testsuite/tests/simplCore/should_compile/T7360.hs
index 2bf31f200a..4da49041f8 100644
--- a/testsuite/tests/simplCore/should_compile/T7360.hs
+++ b/testsuite/tests/simplCore/should_compile/T7360.hs
@@ -6,7 +6,7 @@ module T7360 where
 import GHC.List as L
 
 data Foo = Foo1 | Foo2 | Foo3 !Int
-    
+
 fun1 :: Foo -> ()
 {-# NOINLINE fun1 #-}
 fun1 x = case x of
@@ -14,7 +14,7 @@ fun1 x = case x of
                Foo2 -> ()
                Foo3 {} -> ()
 
-fun2 x = (fun1 Foo1,  -- Keep -ddump-simpl output 
+fun2 x = (fun1 Foo1,  -- Keep -ddump-simpl output
                       -- in a predictable order
          case x of
           [] -> L.length x
diff --git a/testsuite/tests/simplCore/should_compile/T7360.stderr b/testsuite/tests/simplCore/should_compile/T7360.stderr
index b74aee564e..45c88f376e 100644
--- a/testsuite/tests/simplCore/should_compile/T7360.stderr
+++ b/testsuite/tests/simplCore/should_compile/T7360.stderr
@@ -1,7 +1,7 @@
 
 ==================== Tidy Core ====================
 Result size of Tidy Core
-  = {terms: 114, types: 53, coercions: 0, joins: 0/0}
+  = {terms: 106, types: 47, coercions: 0, joins: 0/0}
 
 -- RHS size: {terms: 6, types: 3, coercions: 0, joins: 0/0}
 T7360.$WFoo3 [InlPrag=INLINE[0]] :: Int -> Foo
@@ -25,21 +25,13 @@ fun1 [InlPrag=NOINLINE] :: Foo -> ()
 fun1 = \ (x :: Foo) -> case x of { __DEFAULT -> GHC.Tuple.() }
 
 -- RHS size: {terms: 2, types: 0, coercions: 0, joins: 0/0}
-T7360.fun5 :: ()
+T7360.fun4 :: ()
 [GblId,
  Unf=Unf{Src=<vanilla>, TopLvl=True, Value=False, ConLike=False,
          WorkFree=False, Expandable=False, Guidance=IF_ARGS [] 20 0}]
-T7360.fun5 = fun1 T7360.Foo1
+T7360.fun4 = fun1 T7360.Foo1
 
--- RHS size: {terms: 2, types: 0, coercions: 0, joins: 0/0}
-T7360.fun4 :: Int
-[GblId,
- Cpr=m1,
- Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True,
-         WorkFree=True, Expandable=True, Guidance=IF_ARGS [] 10 20}]
-T7360.fun4 = GHC.Types.I# 0#
-
--- RHS size: {terms: 16, types: 13, coercions: 0, joins: 0/0}
+-- RHS size: {terms: 11, types: 8, coercions: 0, joins: 0/0}
 fun2 :: forall {a}. [a] -> ((), Int)
 [GblId,
  Arity=1,
@@ -48,24 +40,18 @@ fun2 :: forall {a}. [a] -> ((), Int)
  Unf=Unf{Src=InlineStable, TopLvl=True, Value=True, ConLike=True,
          WorkFree=True, Expandable=True,
          Guidance=ALWAYS_IF(arity=1,unsat_ok=True,boring_ok=False)
-         Tmpl= \ (@a) (x [Occ=Once!] :: [a]) ->
-                 (T7360.fun5,
-                  case x of wild [Occ=Once] {
-                    [] -> T7360.fun4;
-                    : _ [Occ=Dead] _ [Occ=Dead] ->
-                      case GHC.List.$wlenAcc @a wild 0# of ww2 [Occ=Once] { __DEFAULT ->
-                      GHC.Types.I# ww2
-                      }
+         Tmpl= \ (@a) (x [Occ=Once] :: [a]) ->
+                 (T7360.fun4,
+                  case x of wild [Occ=Once] { __DEFAULT ->
+                  case GHC.List.$wlenAcc @a wild 0# of ww2 [Occ=Once] { __DEFAULT ->
+                  GHC.Types.I# ww2
+                  }
                   })}]
 fun2
   = \ (@a) (x :: [a]) ->
-      (T7360.fun5,
-       case x of wild {
-         [] -> T7360.fun4;
-         : ds ds1 ->
-           case GHC.List.$wlenAcc @a wild 0# of ww2 { __DEFAULT ->
-           GHC.Types.I# ww2
-           }
+      (T7360.fun4,
+       case GHC.List.$wlenAcc @a x 0# of ww2 { __DEFAULT ->
+       GHC.Types.I# ww2
        })
 
 -- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}