"Arity decrease" warnings when compiling Haddock

I'm getting warnings of the form

WARNING: file compiler/GHC/Core/Op/Simplify/Utils.hs, line 1528
  Arity decrease: lvl_s4yB 3 0 0
  lvl_s4XI
  `cast` (Sym (N:Builder[0])
          :: (forall r. BuildStep r -> BuildStep r) ~R# Builder)
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1289:29 in ghc:Outputable
        warnPprTrace, called at compiler/GHC/Core/Op/Simplify/Utils.hs:1526:11 in ghc:GHC.Core.Op.Simplify.Utils

Having looked at prepareRhs lately, I suspect it has to do that we don't transfer lvl_s4yB's arityInfo on to lvl_s4XI's sanitised_info in the makeTrivialWithInfo case.

As I said in #17673 (comment 267431), we probably want to share the logic with WW, which doesn't have this weakness.