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	Add continuation splitting to Simplify
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When the simplifier finds a 'case', it calls mkDupableAlt
to make the "continuation" (that is, the context of the
case expression) duplicatable, so that it can push it into
the case branches.  This is crucial for the case-of-case
transformation.

But it turns out that it's a bad idea to do that when
the context is "I'm the argument of a strict function".  Consider

	f (case x of { True -> False; False -> True }) arg2

where f is a strict function.  Then we *could* (and were)
transforming to

	let $j a = f a arg2
	in
	case x of { True -> $j False; False -> $j True }

But this is in general a terribly bad thing to do.
See the example in comments with Simplify.mkDupableCont.