Updates to multi-entry.

* Convert Zone to multi-entry.  This was the last of the SSA and SSA2 optimization passes to be ported.  The only remaining SSA/SSA2 modules to be converted to multi-entry are `DirectExp2` and `Restore2`.  However, these modules are not used in the single-entry pipeline.  In fact, these modules are not even included in `sources.cm`/`sources.mlb`.  They appear to have been copied at the time SSA2 was introduced, but never actually converted from SSA to SSA2; indeed, they don't even type-check if included in `sources.cm`/`sources.mlb`.

* Update conversion of SignalCheck to multi-entry.  This fixes all of the signals/threads regression failures.