GHC HEAD panics in add_demands

Test case:

{-# OPAQUE nexti #-}
nexti :: Int -> Int
nexti x = succ x

This is on a branch which includes commit b3be0d18

This is the last output by -dverbose-core2core before the panic:

-- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}
nexti [InlPrag=OPAQUE] :: Int -> Int
[LclIdX,
 Arity=1,
 Unf=Unf{Src=<vanilla>, TopLvl=True,
         Value=True, ConLike=True, WorkFree=True, Expandable=True,
         Guidance=ALWAYS_IF(arity=0,unsat_ok=True,boring_ok=True)}]
nexti = GHC.Enum.$fEnumInt_$csucc

Reminiscent of f5fd4db6 but this time without involving any casts.

@sgraf812 @simonpj Let me know if you want to pick this up. If not I will likely do so.