Update stackMapping to Offset -> Cell

[sadrabt]

Currently stackMapping is implemented as Offset -> Node with cell 0 of the node representing the cell for that offset. This is fine in the local phase since if cell 0 of the stack node gets merged into a different offset of another node, the local graph will track it via the union-find data structure. However, once the graph is cloned, the union-find information is lost and hence stack positions which have been updated to be a cell at an offset different to 0 will be lost.