Perform the ALAP Variable Declaration process.
The process is structured in the following stages:
- We start by collecting all the `Variable`s that need to be assigned in
a `ASTNode`.
- We build a `GenericGraph` where each `ASTNode` is connected with its
child nodes, and also its immediate successor.
- We perform a walk in post order over this graph, and for each
`ASTNode` we decide if the current node could be the ALAP location
where we can declare the `Variable`, i.e., where we dominate all its
uses.
Alessandro Di Federico