The coercion optimizer will take care of it anyway, and the check is
See Trac Trac #14737.
Do the same for pushCoValArg.
Does this patch (alone) have any perf impact?
In D4568#126469, @simonpj wrote:
Yes, a rather big one. See Trac #14737: execution time for compiling Grammar.hs goes down from ~12s to ~8s.