Code generation for unboxed computations about flambda-backend

[Gbury]

Technically, to_cmm pushes the additions down rather than pushing the calls up, but that's a detail, ^^

That being said, from my point of view, the problem is how to decide when to push things down vs when not to, and from what I can tell right now, that answer seems highly dependant on the context: i.e. the number of live values, whether pushing float operations down actually makes some cmm-level optimizations possible, etc... All of these criterion are complex and not very local, and therefore, I don't think to_cmm is the right place to decide what to do: we could envision a scheme where the decision is made by flambda2 and then carried out by to_cmm (if it's easier to do there rather than in simplify).