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Previously CoreToStg would unconditionally discard cases of the form: case unsafeEqualityProof of wild { _ -> rhs } and rather replace the whole thing with `rhs`. However, in some cases (see #18227) the case binder is still live, resulting in unbound occurrences in `rhs`. Fix this by only discarding the case if the case binder is dead. Fixes #18227.
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