This commit adds handling for a corner case of
MergePointeesOfPointerUnion, that wasn't handled before.
Said corner case happens when one of the scalars that gets merged as
part of the DLAStep operations is also itself a pointer to another
scalar that gets merged.
This wasn't accounted for before this commit, but it's actually a valid
scenario that can happen on some inputs.
Before this commit, the DLAStep MergePointeesOfPointerUnion wasn't
really equipped for dealing with LayoutTypeSystemNodes representing
types imported from Model.
There was some code that tried to deal with them but it was mostly an
afterthought and it wasn't robust.
This commit fixes that, and is able to handle a strict superset of the
scenarios envisioned before, while preserving the property that nodes
representing types imported from Model should be preserved (in
particular their size).
Alessandro Di Federico