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	Improve the demand analyser [case]
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1. In the Case case of dmdAnal, I dealt with the case binder in a way that
was both clumsy and pessimistic.  This commit fixes that:

	-- Figure out whether the demand on the case binder is used, and use
	-- that to set the scrut_dmd.  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.
	-- So at one stage I had:
	--	dead_case_bndr		 = isAbsentDmd (idNewDemandInfo case_bndr')
	--	keepity | dead_case_bndr = Drop
	--		| otherwise	 = Keep
	--
	-- But then consider
	--	case x of y { (a,b) -> h y + a }
	-- where h : U(LL) -> T
	-- The above code would compute a Keep for x, since y is not Abs, which is silly
	-- The insight is, of course, that a demand on y is a demand on the
	-- scrutinee, so we need to `both` it with the scrut demand

        scrut_dmd 		 = Seq Drop Now [idNewDemandInfo b | b <- bndrs', isId b]
				   `both`
				   idNewDemandInfo case_bndr'

	-- There used to be a special case for when
	--	ty == TyVarTy tv
	-- (a not-uncommon case) in which case the substitution was dropped.
	-- But the type-tidier changes the print-name of a type variable without
	-- changing the unique, and that led to a bug.   Why?  Pre-tidying, we had
	-- a type {Foo t}, where Foo is a one-method class.  So Foo is really a newtype.
	-- And it happened that t was the type variable of the class.  Post-tiding,


2. 'defer' can be simplified to 'lub Abs', reducing the number of places
where things can go wrong.

3. Add comments