This implements an optimization pass for multi-function procedures.
It optimizes memory reuse by moving destruct instructions up.
For more details see the in-code comment.
In very large fields with many short lived intermediate values, this change
can improve performance 3-4x. Furthermore, in such cases, peak memory
consumption is reduced significantly (e.g. 100x lower peak memory usage).
Differential Revision: https://developer.blender.org/D13548
