Statically estimate execution frequency of Core case alternative

Consider

sum n = case n of
  0 -> 0 :: Int
  _ -> n + sum (n-1)

Having an estimate that says that the recursive path is hotter than the non-recursive path is quite useful to a number of optimisations:

• The Simplifier could decide not to inline in the cold branch, having more code size to spend inlining in the second branch.
• The first branch may need some value boxed, but the second one doesn't. It may be more efficient to unbox the value and re-allocate the box in the first branch.
• Similarly, we could give 'sum' the CPR property, thereby unboxing its result and destroying the sharing of the (then floated out) 0 CAF.
• We may float out a binding from the second alternative, even if that means more allocation if we take the first alternative
• It would allow easy integration of https://github.com/AndreasPK/ghc-proposals/blob/master/proposals/0000-likelihood-annotations.rst, although that appears to be somewhat orthogonal
• (Feel free to add more...)

There's abundant literature on how to come up with such estimates for imperative IRs/control flow graphs. We just have to transfer the knowledge there to multi-way case alternatives.