Improve demand analysis for join points

Consider

   g :: (Int,Int) -> Int
   g (p,q) = p+q

   f :: Int -> Int -> Int
   f x p = g (join j y = (p,y)
              in case x of
                   True  -> j 3
                   False -> j 4)

If j was a vanilla function definition, we'd analyse its body with evalDmd, and think that it was lazy in p.

But for a join point we can do better. We know that j's body (if evaluated at all) will be evaluated with the demand that consumes the entire join-binding, in this case the argument demand from g. Whizzo! g evaluates both components of its arugment pair, so j is strict in p.

So, when analysing a join point, we can analyse its body with the demand from the entire join-binding. Another win for join points!

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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