This function computes the CSV that may be accessed from a call in root.
Before this commit, it assumed that all the load and store had to be
aligned with the underlying variables. This happens to be too strict an
assumption, and it is relaxed in this commit.

When a load or a store is not aligned with the underlying type, this
commit introduces code to mark all the spanned CSV as accessed.
This is information is then attached to the call site in root as
metadata, as it already happened before.