Fix shadowing in mkWwBodies
This bug, exposed by Trac #12562 was very obscure, and has been lurking for a long time. What happened was that, in the worker/wrapper split a tyvar binder for a worker function accidentally shadowed an in-scope term variable that was mentioned in the body of the function It's jolly hard to provoke, so I have not even attempted to make a test case. There's a Note [Freshen WW arguments] to explain. Interestingly, fixing the bug (which meant fresher type variables) revealed a second lurking bug: I'd failed to apply the substitution to the coercion in the second last case of mkWWArgs, which introduces a Cast.
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