Use range in symex instead of merging sets of expressions

This part of the code is executed in all our tests so the fact that all
existing tests pass should show this change works correctly.

The performance difference for this change were evaluate on 143 methods
from tika, which were chosen because analysis time was taking between 1
second and 1 minute for them.
The time of symex step was evaluated alone (options --show-vcc --verbosity 0).

The execution time for the whole of the benchmarks was reduced by 21%
and the average speed up is 14% (the difference between the two means
the optimization seems to have less influence on simpler benchmarks).

Author: romainbrenguier

src/goto-symex/symex_goto.cpp

@@ -16,6 +16,7 @@ Author: Daniel Kroening, kroening@kroening.com
 #include <util/exception_utils.h>
 #include <util/invariant.h>
 #include <util/pointer_offset_size.h>
+#include <util/range.h>
 #include <util/std_expr.h>
 
 #include <analyses/dirty.h>
@@ -357,26 +358,37 @@ void goto_symext::phi_function(
   const statet::goto_statet &goto_state,
   statet &dest_state)
 {
-  // go over all variables to see what changed
-  std::unordered_set<ssa_exprt, irep_hash> variables;
+  auto ssa_of_current_name =
+    [&](const std::pair<irep_idt, std::pair<ssa_exprt, unsigned>> &pair) {
+      return pair.second.first;
+    };
+
+  auto dest_state_names_range =
+    make_range(dest_state.level2.current_names)
+      .filter(
+        [&](const std::pair<irep_idt, std::pair<ssa_exprt, unsigned>> &pair) {
+          // We ignore the identifiers that are already in goto_state names
+          return goto_state.level2_current_names.count(pair.first) == 0;
+        })
+      .map<const ssa_exprt>(ssa_of_current_name);
 
-  goto_state.level2_get_variables(variables);
-  dest_state.level2.get_variables(variables);
+  // go over all variables to see what changed
+  auto all_current_names_range = make_range(goto_state.level2_current_names)
+                                   .map<const ssa_exprt>(ssa_of_current_name)
+                                   .concat(dest_state_names_range);
 
   guardt diff_guard;
-
-  if(!variables.empty())
+  if(!all_current_names_range.empty())
   {
     diff_guard=goto_state.guard;
 
     // this gets the diff between the guards
     diff_guard-=dest_state.guard;
   }
 
-  for(std::unordered_set<ssa_exprt, irep_hash>::const_iterator
-      it=variables.begin();
-      it!=variables.end();
-      it++)
+  for(auto it = all_current_names_range.begin();
+      it != all_current_names_range.end();
+      ++it)
   {
     const irep_idt l1_identifier=it->get_identifier();
     const irep_idt &obj_identifier=it->get_object_name();