Symex phi function: merge global variables like any other

Now that the front-ends are required to explicitly assign a nondet value where that
is intended (e.g. an undefined extern global), rather than leaving an undefined value,
there is no need to special-case merging them.

Author: smowton

src/goto-symex/symex_goto.cpp

@@ -520,14 +520,10 @@ static void merge_names(
     rhs = goto_state_rhs;
   else if(goto_state.guard.is_false())
     rhs = dest_state_rhs;
-  else if(goto_count == 0 && !symbol.is_static_lifetime)
-  {
+  else if(goto_count == 0)
     rhs = dest_state_rhs;
-  }
-  else if(dest_count == 0 && !symbol.is_static_lifetime)
-  {
+  else if(dest_count == 0)
     rhs = goto_state_rhs;
-  }
   else
   {
     rhs = if_exprt(diff_guard.as_expr(), goto_state_rhs, dest_state_rhs);