This patch clears up a long-standing wart.   For some time it's been the
case that 
	the RHS of a non-recursive let can be unlifed iff 
	the RHS is ok-for-speculation

This patch extends the invariant to the argument of an App, and 
establishes it by the smart constructors mkDsApp, mkDsApps in the desugarer.

Once established, it should be maintained by the optimiser.

This tides up some awkward cases, notably in exprIsHNF, and I think it
fixes a outright strictness bug in Simplify.prepareRhs.