On N127-001, an optimisation of the fast WP was introduced, based on
marking states that contain variables that need to be refreshed when
merging them. However, this "marked" status was incorrectly reset to
false when merging states, even though some of those "unfresh" variables
could survive the merge. The consequence was that the WP was reusing
variables too aggressively, which would result in incorrect VCs.

We now compute the marked status of a merged state from the states to be
merged: if one of them is marked, the merged state is marked too.

mlw_wp.ml:
(merge): synthetize marked status from input states

Change-Id: Ifb02c54fca58137c31762469e48b59e7c907b995