Move size variables used in allocation statements upwards in the bodies of a program to enable more memory block reuses.

This should be run *before* the reuse pass, as it enables more optimisations. Specifically, it helps with reusing memory whose size needs to be changed to be the maximum of itself and another size -- and so, that other size needs to have been hoisted so that is in scope at that point. This module hoists all sizes as much as possible.