Note [Checking for representation polymorphism]

We ordinarily want to check for bad representation polymorphism. See
Note [Representation polymorphism invariants] in GHC.Core. However, we do *not*
want to do this in a compulsory unfolding. Compulsory unfoldings arise
only internally, for things like newtype wrappers, dictionaries, and
(notably) unsafeCoerce#. These might legitimately be representation-polymorphic;
indeed representation-polymorphic unfoldings are a primary reason for the
very existence of compulsory unfoldings (we can't compile code for
the original, representation-polymorphic, binding).

It is vitally important that we do representation polymorphism checks *after*
performing the unfolding, but not beforehand. This is all safe because
we will check any unfolding after it has been unfolded; checking the
unfolding beforehand is merely an optimization, and one that actively
hurts us here.