introduce non-const instructiont::get_condition()

This will eventually allow us to protect the guard data member.

Author: Daniel Kroening

src/analyses/constant_propagator.cpp

@@ -756,11 +756,12 @@ void constant_propagator_ait::replace(
       continue;
 
     replace_types_rec(d.values.replace_const, it->code);
-    replace_types_rec(d.values.replace_const, it->guard);
 
     if(it->is_goto() || it->is_assume() || it->is_assert())
     {
-      constant_propagator_domaint::partial_evaluate(d.values, it->guard, ns);
+      replace_types_rec(d.values.replace_const, it->get_condition());
+      constant_propagator_domaint::partial_evaluate(
+        d.values, it->get_condition(), ns);
     }
     else if(it->is_assign())
     {

src/goto-programs/goto_program.h

@@ -287,6 +287,13 @@ class goto_programt
       return guard;
     }
 
+    /// Get the condition of gotos, assume, assert
+    exprt &get_condition()
+    {
+      PRECONDITION(has_condition());
+      return guard;
+    }
+
     // The below will eventually become a single target only.
     /// The target for gotos and for start_thread nodes
     typedef std::list<instructiont>::iterator targett;