There were two bugs in the Etry case for fastwp:

- soundness issue: the case for the normal exit was wrong.  It tried to
  merge the states iteratively by pairs, which is wrong. Instead, states
  need to be merged at once. Given the traversal that was done, it wasn't
  easily fixable, so a rewrite of the entire Etry case was necessary.
- completeness issue: some information was dropped for the exceptional
  exits. Some code is required to recover this information.

* mlw_wp.ml
(fast_wp_desc): fix Etry case

Change-Id: Ic39045baa145b3933312f4d2499582a9251b2f6c
(cherry picked from commit c50300b6f4aa193e008492a5af719ee03cd72211)