A major demand-analyser fix, which made it say something was
absent when it wasn't at all.  Here's the comment from the
Case equation of dmdAnal.

	-- Figure out whether the case binder is used, and use
	-- that to set the keepity of the demand.  This is utterly essential.
	-- Consider	f x = case x of y { (a,b) -> k y a }
	-- If we just take scrut_demand = U(L,A), then we won't pass x to the
	-- worker, so the worker will rebuild
	--	x = (a, absent-error)
	-- and that'll crash.