Here's the obscure problem:
-- However, we also want to allow an assignment to be generated
-- in the case when the types are compatible, because this allows
-- some slightly-dodgy but occasionally-useful casts to be used,
-- such as in RtClosureInspect where we cast an HValue to a MutVar#
-- so we can print out the contents of the MutVar#.  If we generate
-- code that enters the HValue, then we'll get a runtime panic, because
-- the HValue really is a MutVar#.  The types are compatible though,
-- so we can just generate an assignment.