Use Reductions to keep track of rewritings

We define Reduction = Reduction Coercion !Type.
A reduction of the form 'Reduction co new_ty' witnesses an
equality ty ~co~> new_ty.
That is, the rewriting happens left-to-right: the right-hand-side
type of the coercion is the rewritten type, and the left-hand-side
type the original type.
Sticking to this convention makes the codebase more consistent,
helping to avoid certain applications of SymCo.

This replaces the parts of the codebase which represented reductions as
pairs, (Coercion,Type) or (Type,Coercion).

Reduction being strict in the Type argument improves performance
in some programs that rewrite many type families (such as T9872).

Fixes #20161

-------------------------
Metric Decrease:
  T5321Fun
    T9872a
    T9872b
    T9872c
    T9872d
-------------------------

1 files changed, 2 insertions, 1 deletions

diff --git a/compiler/GHC/Core/Opt/WorkWrap/Utils.hs b/compiler/GHC/Core/Opt/WorkWrap/Utils.hs
index c3e708c60d..beafa01b1c 100644
--- a/compiler/GHC/Core/Opt/WorkWrap/Utils.hs
+++ b/compiler/GHC/Core/Opt/WorkWrap/Utils.hs
@@ -30,6 +30,7 @@ import GHC.Core.Type
 import GHC.Core.Multiplicity
 import GHC.Core.Predicate ( isClassPred )
 import GHC.Core.Coercion
+import GHC.Core.Reduction
 import GHC.Core.FamInstEnv
 import GHC.Core.TyCon
 import GHC.Core.TyCon.RecWalk
@@ -1239,7 +1240,7 @@ findTypeShape fam_envs ty
        = TsUnk
 
     go_tc rec_tc tc tc_args
-       | Just (_, rhs, _) <- topReduceTyFamApp_maybe fam_envs tc tc_args
+       | Just (HetReduction (Reduction _ rhs) _) <- topReduceTyFamApp_maybe fam_envs tc tc_args
        = go rec_tc rhs
 
        | Just con <- tyConSingleAlgDataCon_maybe tc