Disable speculative evaluation in coreprep in ghc 9.4, 9.6, 9.8

Speculative evaluation was found to cause a large increase in
allocations in certain situations. We disable it for now.

Author: luite

overlays/bootstrap.nix

@@ -84,6 +84,7 @@ in {
                 # will be applied to most versions of the GHC anyway (reordering the patches
                 # results in rebuilds of GHC and reduces sharing in /nix/store).
                 in always                    ./patches/ghc/dll-loader-8.4.2.patch                                # https://gitlab.haskell.org/ghc/ghc/merge_requests/949  -- open
+                ++ fromUntil "9.4"   "9.10"  ./patches/ghc/ghc-9.4-9.6-9.8-disable-coreprep-spec-eval.patch      # causes more allocations
                 ++ until             "9.2"   ./patches/ghc/ghc-8.4.3-Cabal2201-no-hackage-tests.patch            # ?
                 ++ until             "9.2"   ./patches/ghc/cabal-host.patch                                      # https://github.com/haskell/cabal/issues/5887
                 ++ fromUntil "9.2"   "9.4"   ./patches/ghc/ghc-9.2-cabal-host.patch                              # https://github.com/haskell/cabal/issues/5887

overlays/patches/ghc/ghc-9.4-9.6-9.8-disable-coreprep-spec-eval.patch

@@ -0,0 +1,13 @@
+diff --git a/compiler/GHC/CoreToStg/Prep.hs b/compiler/GHC/CoreToStg/Prep.hs
+index c463f41121..58b1d957c8 100644
+--- a/compiler/GHC/CoreToStg/Prep.hs
++++ b/compiler/GHC/CoreToStg/Prep.hs
+@@ -1759,7 +1759,7 @@ data OkToSpec
+ 
+ mkFloat :: CorePrepEnv -> Demand -> Bool -> Id -> CpeRhs -> FloatingBind
+ mkFloat env dmd is_unlifted bndr rhs
+-  | is_strict || ok_for_spec -- See Note [Speculative evaluation]
++  | is_strict {-|| ok_for_spec-} -- See Note [Speculative evaluation]
+   , not is_hnf  = FloatCase rhs bndr DEFAULT [] ok_for_spec
+     -- Don't make a case for a HNF binding, even if it's strict
+     -- Otherwise we get  case (\x -> e) of ...!