Two small improvements in the Simplifier

As #20941 describes, this patch implements a couple of small
fixes to the Simplifier.  They make a difference principally
with -O0, so few people will notice.  But with -O0 they can
reduce the number of Simplifer iterations.

* In occurrence analysis we avoid making x = (a,b) into a loop breaker
  because we want to be able to inline x, or (more likely) do
  case-elimination. But HEAD does not treat
      x = let y = blah in (a,b)
  in the same way.  We should though, because we are going to float
  that y=blah out of the x-binding.  A one-line fix in OccurAnal.

* The crucial function exprIsConApp_maybe uses getUnfoldingInRuleMatch
  (rightly) but the latter was deeply strange.  In HEAD, if
  rule-rewriting was off (-O0) we only looked inside stable
  unfoldings. Very stupid.  The patch simplifies.

* I also noticed that in simplStableUnfolding we were failing to
  delete the DFun binders from the usage.  So I added that.

Practically zero perf change across the board, except that we get more
compiler allocation in T3064 (which is compiled with -O0). There's a
good reason: we get better code.  But there are lots of other small
compiler allocation decreases:

Metrics: compile_time/bytes allocated
---------------------
                                   Baseline
               Test    Metric         value     New value Change
-----------------------------------------------------------------
  PmSeriesG(normal) ghc/alloc    44,260,817    44,184,920  -0.2%
  PmSeriesS(normal) ghc/alloc    52,967,392    52,891,632  -0.1%
  PmSeriesT(normal) ghc/alloc    75,498,220    75,421,968  -0.1%
  PmSeriesV(normal) ghc/alloc    52,341,849    52,265,768  -0.1%
     T10421(normal) ghc/alloc   109,702,291   109,626,024  -0.1%
    T10421a(normal) ghc/alloc    76,888,308    76,809,896  -0.1%
     T10858(normal) ghc/alloc   125,149,038   125,073,648  -0.1%
     T11276(normal) ghc/alloc    94,159,364    94,081,640  -0.1%
    T11303b(normal) ghc/alloc    40,230,059    40,154,368  -0.2%
     T11822(normal) ghc/alloc   107,424,540   107,346,088  -0.1%
     T12150(optasm) ghc/alloc    76,486,339    76,426,152  -0.1%
     T12234(optasm) ghc/alloc    55,585,046    55,507,352  -0.1%
     T12425(optasm) ghc/alloc    88,343,288    88,265,312  -0.1%
     T13035(normal) ghc/alloc    98,919,768    98,845,600  -0.1%
 T13253-spj(normal) ghc/alloc   121,002,153   120,851,040  -0.1%
     T16190(normal) ghc/alloc   290,313,131   290,074,152  -0.1%
     T16875(normal) ghc/alloc    34,756,121    34,681,440  -0.2%
    T17836b(normal) ghc/alloc    45,198,100    45,120,288  -0.2%
     T17977(normal) ghc/alloc    39,479,952    39,404,112  -0.2%
    T17977b(normal) ghc/alloc    37,213,035    37,137,728  -0.2%
     T18140(normal) ghc/alloc    79,430,588    79,350,680  -0.1%
     T18282(normal) ghc/alloc   128,303,182   128,225,384  -0.1%
     T18304(normal) ghc/alloc    84,904,713    84,831,952  -0.1%
     T18923(normal) ghc/alloc    66,817,241    66,731,984  -0.1%
     T20049(normal) ghc/alloc    86,188,024    86,107,920  -0.1%
      T5837(normal) ghc/alloc    35,540,598    35,464,568  -0.2%
      T6048(optasm) ghc/alloc    99,812,171    99,736,032  -0.1%
      T9198(normal) ghc/alloc    46,380,270    46,304,984  -0.2%

          geo. mean                                        -0.0%

Metric Increase:
    T3064

2 files changed, 9 insertions, 8 deletions

diff --git a/compiler/GHC/Core/Opt/OccurAnal.hs b/compiler/GHC/Core/Opt/OccurAnal.hs
index bfabae9f51..c5e4dee1a0 100644
--- a/compiler/GHC/Core/Opt/OccurAnal.hs
+++ b/compiler/GHC/Core/Opt/OccurAnal.hs
@@ -1586,7 +1586,9 @@ nodeScore !env new_bndr lb_deps
     is_con_app (App f _)  = is_con_app f
     is_con_app (Lam _ e)  = is_con_app e
     is_con_app (Tick _ e) = is_con_app e
-    is_con_app _          = False
+    is_con_app (Let _ e)  = is_con_app e  -- let x = let y = blah in (a,b)
+    is_con_app _          = False         -- We will float the y out, so treat
+                                          -- the x-binding as a con-app (#20941)
 
 maxExprSize :: Int
 maxExprSize = 20  -- Rather arbitrary
@@ -1935,6 +1937,9 @@ occAnalUnfolding !env is_rec mb_join_arity unf
           (WithUsageDetails usage args') = occAnalList env' args
           final_usage     = markAllManyNonTail (delDetailsList usage bndrs)
                             `addLamCoVarOccs` bndrs
+                            `delDetailsList` bndrs
+              -- delDetailsList; no need to use tagLamBinders because we
+              -- never inline DFuns so the occ-info on binders doesn't matter
 
       unf -> WithUsageDetails emptyDetails unf
 
diff --git a/compiler/GHC/Core/Opt/Simplify/Utils.hs b/compiler/GHC/Core/Opt/Simplify/Utils.hs
index 409f3176eb..13a8a1b853 100644
--- a/compiler/GHC/Core/Opt/Simplify/Utils.hs
+++ b/compiler/GHC/Core/Opt/Simplify/Utils.hs
@@ -1130,13 +1130,9 @@ getUnfoldingInRuleMatch env
     mode = getMode env
     id_unf id | unf_is_active id = idUnfolding id
               | otherwise        = NoUnfolding
-    unf_is_active id
-     | not (sm_rules mode) = -- active_unfolding_minimal id
-                             isStableUnfolding (realIdUnfolding id)
-        -- Do we even need to test this?  I think this InScopeEnv
-        -- is only consulted if activeRule returns True, which
-        -- never happens if sm_rules is False
-     | otherwise           = isActive (sm_phase mode) (idInlineActivation id)
+    unf_is_active id = isActive (sm_phase mode) (idInlineActivation id)
+       -- When sm_rules was off we used to test for a /stable/ unfolding,
+       -- but that seems wrong (#20941)
 
 ----------------------
 activeRule :: SimplMode -> Activation -> Bool