Note [nospecId magic]

The 'nospec' magic Id is used to ensure to make a value opaque to the typeclass
specialiser. In CorePrep, we inline 'nospec', turning (nospec e) into e.
Note that this happens *after* unfoldings are exposed in the interface file.
This is crucial: otherwise, we could import an unfolding in which
'nospec' has been inlined (= erased), and we would lose the benefit.

'nospec' is used:

* In the implementation of 'withDict': we insert 'nospec' so that the
  typeclass specialiser doesn't assume any two evidence terms of the
  same type are equal. See Note [withDict] in GHC.Tc.Instance.Class,
  and see test case T21575b for an example.

* To defeat the specialiser when we have incoherent instances.
  See Note [Coherence and specialisation: overview] in GHC.Core.InstEnv.