Useful RULEs are being discarded

(Side note: This ticket is essentially the same as #18532 (closed).)

Note [Trimming auto rules] in GHC.Core.Iface.Tidy describes how auto-generated specialisation rules are discarded. But this discards way more rules than I thought. Consider

   f :: Num a => a -> a
   f x = ...f....   -- Perhpas big

   foo = f@Int...

The specialiser will specialise f and, if the specialised version is strict, we'll get

   $w$sf :: Int# -> Int#
   $w$sf x = ...$w$sf...   -- Perhaps big

   $sf :: Int -> Int
   $sf x = case x of I# x' -> $w$sf x'

   f x = ....f...;
   {-# RULE "SPEC" forall d. f @Int d = $sf

   foo = ....$w$sf...

Now the only thing keeping $sf alive is that RULE, so according to See Note [Trimming auto rules] we'll drop the RULE. But that's a crying shame because $sf is small, so not much code is retained if we keep it; and the payoff is that importing modules get to see this jolly good RULE.

I think we should measure the effect on binary size of being less aggressive about discarding RULES.