More precise ref_to_var

[chambart]

A bit smarter ref_to_var pass could get rid of the block allocations in:

let f a b =
  let r = ref (a, b) in
  while true do
    ...
    let (x, y) = !r in
    ...
    r := (x', y')
  done

It may be possible to do some more for non tag 0 blocks, but this involves adding a new primitive for allocating a block with a not statically known tag.

Another way to do that may be to go the CPS way and do the optimisation on that representation.

[mshinwell]

That new primitive might cause problems for Spacetime, if we extend the profinfo space by assuming that blocks with different tags were allocated at different locations.