Make rewrite rules "win" over inliningwip/T21851-rule-win

If a rewrite rule and a rewrite rule compete in the simplifier, this
patch makes sure that the rewrite rule "win".  That is, in general
a bit fragile, but it's a huge help when making specialisation work
reliably, as #21851 and #22097 showed.

The change is fairly straightforwad, and documented in
   Note [Rewrite rules and inlining]
in GHC.Core.Opt.Simplify.Iteration.

Compile-times change, up and down a bit -- in some cases because
we get better specialisation.  But the payoff (more reliable
specialisation) is large.

Metrics: compile_time/bytes allocated
-----------------------------------------------
    T10421(normal)   +3.7% BAD
   T10421a(normal)   +5.5%
    T13253(normal)   +1.3%
      T14052(ghci)   +1.8%
    T15304(normal)   -1.4%
    T16577(normal)   +3.1% BAD
    T17516(normal)   +2.3%
    T17836(normal)   -1.9%
    T18223(normal)   -1.8%
     T8095(normal)   -1.3%
     T9961(normal)   +2.5% BAD

         geo. mean   +0.0%
         minimum     -1.9%
         maximum     +5.5%

Nofib results are (bytes allocated)

+-------------------------------++----------+
|                               ||tsv (rel) |
+===============================++==========+
|           imaginary/paraffins ||   +0.27% |
|                imaginary/rfib ||   -0.04% |
|                     real/anna ||   +0.02% |
|                      real/fem ||   -0.04% |
|                    real/fluid ||   +1.68% |
|                   real/gamteb ||   -0.34% |
|                       real/gg ||   +1.54% |
|                   real/hidden ||   -0.01% |
|                      real/hpg ||   -0.03% |
|                    real/infer ||   -0.03% |
|                   real/prolog ||   +0.02% |
|                  real/veritas ||   -0.47% |
|       shootout/fannkuch-redux ||   -0.03% |
|         shootout/k-nucleotide ||   -0.02% |
|               shootout/n-body ||   -0.06% |
|        shootout/spectral-norm ||   -0.01% |
|         spectral/cryptarithm2 ||   +1.25% |
|             spectral/fibheaps ||  +18.33% |
|           spectral/last-piece ||   -0.34% |
+===============================++==========+
|                     geom mean ||   +0.17% |

There are extensive notes in !8897 about the regressions.
Briefly

* fibheaps: there was a very delicately balanced inlining that
  tipped over the wrong way after this change.

* cryptarithm2 and paraffins are caused by #22274, which is
  a separate issue really.  (I.e. the right fix is *not* to
  make inlining "win" over rules.)

So I'm accepting these changes

Metric Increase:
    T10421
    T16577
    T9961

1 files changed, 4 insertions, 1 deletions

diff --git a/compiler/GHC/Utils/Monad.hs b/compiler/GHC/Utils/Monad.hs
index 5366d12dca..6f93ac2c27 100644
--- a/compiler/GHC/Utils/Monad.hs
+++ b/compiler/GHC/Utils/Monad.hs
@@ -163,7 +163,10 @@ mapSndM = traverse . traverse
 -- | Monadic version of concatMap
 concatMapM :: (Monad m, Traversable f) => (a -> m [b]) -> f a -> m [b]
 concatMapM f xs = liftM concat (mapM f xs)
-{-# SPECIALIZE concatMapM :: (Monad m) => (a -> m [b]) -> [a] -> m [b] #-}
+{-# INLINE concatMapM #-}
+-- It's better to inline to inline this than to specialise
+--     concatMapM :: (Monad m) => (a -> m [b]) -> [a] -> m [b]
+-- Inlining cuts compiler allocation by around 1%
 
 -- | Applicative version of mapMaybe
 mapMaybeM :: Applicative m => (a -> m (Maybe b)) -> [a] -> m [b]