This long-standing bug in CoreUtils.combineIdenticalAlts
was shown up by Trac #11172. The effect was that it returned
a correct set of alternatives, but a bogus set of "impossible
default constructors".  That meant that we subsequently
removed all the alternatives from a case, and hence ended
up with a bogusly empty case that should not have been empty.

See Note [Care with impossible-constructors when
combining alternatives] in CoreUtils.