Better eta-expansion (again) and don't specilise DFuns

This patch fixes #18223, which made GHC generate an exponential
amount of code.  There are three quite separate changes in here

1.  Re-engineer eta-expansion (again).  The eta-expander was
    generating lots of intermediate stuff, which could be optimised
    away, but which choked the simplifier meanwhile.  Relatively
    easy to kill it off at source.

    See Note [The EtaInfo mechanism] in GHC.Core.Opt.Arity.
    The main new thing is the use of pushCoArg in getArg_maybe.

2.  Stop Specialise specalising DFuns.  This is the cause of a huge
    (and utterly unnecessary) blowup in program size in #18223.
    See Note [Do not specialise DFuns] in GHC.Core.Opt.Specialise.

    I also refactored the Specialise monad a bit... it was silly,
    because it passed on unchanging values as if they were mutable
    state.

3.  Do an extra Simplifer run, after SpecConstra and before
    late-Specialise.  I found (investigating perf/compiler/T16473)
    that failing to do this was crippling *both* SpecConstr *and*
    Specialise.  See Note [Simplify after SpecConstr] in
    GHC.Core.Opt.Pipeline.

    This change does mean an extra run of the Simplifier, but only
    with -O2, and I think that's acceptable.

    T16473 allocates *three* times less with this change.  (I changed
    it to check runtime rather than compile time.)

Some smaller consequences

* I moved pushCoercion, pushCoArg and friends from SimpleOpt
  to Arity, because it was needed by the new etaInfoApp.

  And pushCoValArg now returns a MCoercion rather than Coercion for
  the argument Coercion.

* A minor, incidental improvement to Core pretty-printing

This does fix #18223, (which was otherwise uncompilable. Hooray.  But
there is still a big intermediate because there are some very deeply
nested types in that program.

Modest reductions in compile-time allocation on a couple of benchmarks
    T12425     -2.0%
    T13253    -10.3%

Metric increase with -O2, due to extra simplifier run
    T9233     +5.8%
    T12227    +1.8%
    T15630    +5.0%

There is a spurious apparent increase on heap residency on T9630,
on some architectures at least.  I tried it with -G1 and the residency
is essentially unchanged.

Metric Increase
    T9233
    T12227
    T9630

Metric Decrease
    T12425
    T13253

1 files changed, 19 insertions, 21 deletions

diff --git a/testsuite/tests/stranal/should_compile/T18122.stderr b/testsuite/tests/stranal/should_compile/T18122.stderr
index ff18350279..f94751fb55 100644
--- a/testsuite/tests/stranal/should_compile/T18122.stderr
+++ b/testsuite/tests/stranal/should_compile/T18122.stderr
@@ -13,9 +13,8 @@ Lib.$trModule4 = "main"#
 -- RHS size: {terms: 2, types: 0, coercions: 0, joins: 0/0}
 Lib.$trModule3 :: GHC.Types.TrName
 [GblId,
- Cpr=m1,
  Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True,
-         WorkFree=True, Expandable=True, Guidance=IF_ARGS [] 10 20}]
+         WorkFree=True, Expandable=True, Guidance=IF_ARGS [] 10 10}]
 Lib.$trModule3 = GHC.Types.TrNameS Lib.$trModule4
 
 -- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}
@@ -28,27 +27,25 @@ Lib.$trModule2 = "Lib"#
 -- RHS size: {terms: 2, types: 0, coercions: 0, joins: 0/0}
 Lib.$trModule1 :: GHC.Types.TrName
 [GblId,
- Cpr=m1,
  Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True,
-         WorkFree=True, Expandable=True, Guidance=IF_ARGS [] 10 20}]
+         WorkFree=True, Expandable=True, Guidance=IF_ARGS [] 10 10}]
 Lib.$trModule1 = GHC.Types.TrNameS Lib.$trModule2
 
 -- RHS size: {terms: 3, types: 0, coercions: 0, joins: 0/0}
 Lib.$trModule :: GHC.Types.Module
 [GblId,
- Cpr=m1,
  Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True,
-         WorkFree=True, Expandable=True, Guidance=IF_ARGS [] 10 30}]
+         WorkFree=True, Expandable=True, Guidance=IF_ARGS [] 10 10}]
 Lib.$trModule = GHC.Types.Module Lib.$trModule3 Lib.$trModule1
 
 -- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}
 Lib.$wfoo [InlPrag=NOINLINE]
   :: GHC.Prim.Int# -> GHC.Prim.Int# -> GHC.Prim.Int#
 [GblId, Arity=2, Str=<L,U><L,U>, Unf=OtherCon []]
-Lib.$wfoo = (GHC.Prim.+#)
+Lib.$wfoo = GHC.Prim.+#
 
 -- RHS size: {terms: 18, types: 14, coercions: 0, joins: 0/0}
-foo [InlPrag=NOUSERINLINE[0]] :: (Int, Int) -> Int -> Int
+foo [InlPrag=NOUSERINLINE[final]] :: (Int, Int) -> Int -> Int
 [GblId,
  Arity=2,
  Str=<S(SL),1*U(1*U(U),A)><S,1*U(U)>,
@@ -56,24 +53,25 @@ foo [InlPrag=NOUSERINLINE[0]] :: (Int, Int) -> Int -> Int
  Unf=Unf{Src=InlineStable, TopLvl=True, Value=True, ConLike=True,
          WorkFree=True, Expandable=True,
          Guidance=ALWAYS_IF(arity=2,unsat_ok=True,boring_ok=False)
-         Tmpl= \ (w_sHs [Occ=Once!] :: (Int, Int))
-                 (w1_sHt [Occ=Once!] :: Int) ->
-                 case w_sHs of { (ww1_sHw [Occ=Once!], _ [Occ=Dead]) ->
-                 case ww1_sHw of { GHC.Types.I# ww4_sHz [Occ=Once] ->
-                 case w1_sHt of { GHC.Types.I# ww6_sHF [Occ=Once] ->
-                 case Lib.$wfoo ww4_sHz ww6_sHF of ww7_sHJ [Occ=Once] { __DEFAULT ->
-                 GHC.Types.I# ww7_sHJ
+         Tmpl= \ (w_sEf [Occ=Once1!] :: (Int, Int))
+                 (w1_sEg [Occ=Once1!] :: Int) ->
+                 case w_sEf of { (ww1_sEj [Occ=Once1!], _ [Occ=Dead]) ->
+                 case ww1_sEj of { GHC.Types.I# ww4_sEm [Occ=Once1] ->
+                 case w1_sEg of { GHC.Types.I# ww6_sEs [Occ=Once1] ->
+                 case Lib.$wfoo ww4_sEm ww6_sEs of ww7_sEw [Occ=Once1]
+                 { __DEFAULT ->
+                 GHC.Types.I# ww7_sEw
                  }
                  }
                  }
                  }}]
 foo
-  = \ (w_sHs :: (Int, Int)) (w1_sHt :: Int) ->
-      case w_sHs of { (ww1_sHw, ww2_sHB) ->
-      case ww1_sHw of { GHC.Types.I# ww4_sHz ->
-      case w1_sHt of { GHC.Types.I# ww6_sHF ->
-      case Lib.$wfoo ww4_sHz ww6_sHF of ww7_sHJ { __DEFAULT ->
-      GHC.Types.I# ww7_sHJ
+  = \ (w_sEf :: (Int, Int)) (w1_sEg :: Int) ->
+      case w_sEf of { (ww1_sEj, ww2_sEo) ->
+      case ww1_sEj of { GHC.Types.I# ww4_sEm ->
+      case w1_sEg of { GHC.Types.I# ww6_sEs ->
+      case Lib.$wfoo ww4_sEm ww6_sEs of ww7_sEw { __DEFAULT ->
+      GHC.Types.I# ww7_sEw
       }
       }
       }