Some places in the compiler assume arity of an `Id` is the same as the depth of its `StrictSig`

That assumption isn't always upheld; see Note [idArity varies independently of dmdTypeDepth] in DmdAnal.

Rather than assuming that indeed in all cases where we assume that idArity matches as an unchecked precondition, we should use functions like appIsBottom (appIsDeadEnd after !3014 (closed)). Here is an inexhaustive list of places in the code where we don't account for arity-DmdType-depth mismatches:

• Id.isBottomingId: Just checks isBottomingSig, which doesn't do any arity checks. Probably use appIsBottom sig (idArity id) instead, or pass the arity to check to isBottomingId. This function probably poses the biggest "thread".
• Hs.Core.Utils.exprIsBottom (maybe also exprIsDeadEnd after !3014 (closed), you get the idea): In the Var id case, it checks whether isBottomingId id and then checks whether the number of incoming arguments is >= idArity id. I think it should compare to the DmdType depth instead. For example, DmdType depth might be 2 but idArity might be 1. I don't know if there is such a scenario, but if there is, then it means chaos.
• Hs.Core.Utils.isCheapApp: Doesn't consider arity/n_val_args at all. Even Note [isCheapApp: bottoming functions] says that it's unclear why we have this special case. But I dug up 9be18ea4 and apparently, this check used to be in a function called notRedex, that then got torn apart (407c11b8) into isWorkFreeApp and the isBottomingId check, leaving that Note behind. I think the gist is that a partial application is always a cheap app, and as soon as we saturate the StrictSig, we bottom out, which we also consider cheap in that it doesn't have to be shared. BUT: What if we had n_val_args == idArity fn == 1, but the StrictSig had depth 2? Then certainly that application is not cheap! So this should also call appIsBottom (idStrictness fn) n_val_args.
• Hs.Core.Op.FloatOut.floatBind: Looks at isBottomingId, the eta-expands with idArity. Should probably call appIsBottom (idStrictness var) (idArity var).
• Other usages in LiberateCase, Simplify, SetLevels