Make small INLINE functions behave properly

Simon writes:

Currently we check for a type arg rather than isTyCoArg.   This in turn
makes INLINE things look bigger than they should be, and stops them
being inlined into boring contexts when they perfectly well could be.
E.g.

    f x = g <refl> x
    {-# INLINE g #-}

    ... (map (f x) xs) ...

The context is boring, so don't inline unconditionally.  But f's RHS is
no bigger than its call, provided you realise that the coercion argument
is ultimately cost-free.

This happens in practice for $WHRefl. It's not a big deal: at most it
means we have an extra function call overhead.  But it's untidy, and
actually worse than what happens without an INLINE pragma.

Fixes #17182

This makes 0.0% change in nofib binary sizes.

compiler/coreSyn/CoreUnfold.hs

@@ -441,10 +441,10 @@ inlineBoringOk e
     go :: Int -> CoreExpr -> Bool
     go credit (Lam x e) | isId x           = go (credit+1) e
                         | otherwise        = go credit e
-    go credit (App f (Type {}))            = go credit f
-    go credit (App f a) | credit > 0
+    go credit (App f a) | isTyCoArg a      = go credit f
+                        | credit > 0
                         , exprIsTrivial a  = go (credit-1) f
-    go credit (Tick _ e)                 = go credit e -- dubious
+    go credit (Tick _ e)                   = go credit e -- dubious
     go credit (Cast e _)                   = go credit e
     go _      (Var {})                     = boringCxtOk
     go _      _                            = boringCxtNotOk

testsuite/tests/deSugar/should_compile/T2431.stderr

@@ -10,7 +10,7 @@ T2431.$WRefl [InlPrag=INLINE[0]] :: forall a. a :~: a
  Str=m,
  Unf=Unf{Src=InlineStable, TopLvl=True, Value=True, ConLike=True,
          WorkFree=True, Expandable=True,
-         Guidance=ALWAYS_IF(arity=0,unsat_ok=True,boring_ok=False)
+         Guidance=ALWAYS_IF(arity=0,unsat_ok=True,boring_ok=True)
          Tmpl= \ (@ a) -> T2431.Refl @ a @ a @~ (<a>_N :: a GHC.Prim.~# a)}]
 T2431.$WRefl
   = \ (@ a) -> T2431.Refl @ a @ a @~ (<a>_N :: a GHC.Prim.~# a)
@@ -110,3 +110,6 @@ T2431.$tc'Refl
       $tc'Refl2
       1#
       $krep3
+
+
+