WorkWrap: "splitFun" warning triggers when join point arity is lower than idArity

When compiling GHC/Iface/Syntax.hs with debug output I see the warning

WARNING:
  splitFun
  fail_shak [A]
  Call stack:
      CallStack (from HasCallStack):
        warnPprTrace, called at compiler/GHC/Core/Opt/WorkWrap.hs:761:5 in ghc:GHC.Core.Opt.WorkWrap

And indeed

  join {
     fail_shak [Dmd=MC(1,C(S,L))] :: (# #) -> SDoc
     [LclId[JoinId(1)(Nothing)],
      Arity=2,
      Str=<A> {de7k->SL shaa->SL shtY->SL},
      Unf=Unf{Src=<vanilla>, TopLvl=False,
              Value=True, ConLike=True, WorkFree=True,
              Expandable=True,
              Guidance=IF_ARGS [0] 410 60}]
     fail_shak _ [Occ=Dead, Dmd=A, OS=OneShot]
       = (\ (v_afo4 [OS=OneShot] :: SDocContext) -> ...
  } in ... jump fail_shak (# #); ...

So the whole expression has arity 1 to begin with.

I don't think this is harmful. We can just WW fail for arity 1. Perhaps we should relax the guard to dmds <= idArity for join points.