goto-symex can safely propagate with_exprt

We would only deem with_exprt constant when all their operands are
constant, which means that the array operand must be an array or
array_of expression, and both update indices as well as update values
must be constant. In most such cases the simplifier would rewrite the
with_exprt anyway. The exception to this is an index that cannot
immediately be evaluated to a numeric constant, as is the case with,
e.g., pointer_object expressions.

Fixes: diffblue#3095

Author: tautschnig

src/goto-symex/goto_symex_is_constant.h

@@ -40,21 +40,6 @@ class goto_symex_is_constantt : public is_constantt
 
       return !found_non_constant;
     }
-    else if(expr.id() == ID_with)
-    {
-      // this is bad
-#if 0
-      for(const auto &op : expr.operands())
-      {
-        if(!is_constant(op))
-          return false;
-      }
-
-      return true;
-#else
-      return false;
-#endif
-    }
 
     return is_constantt::is_constant(expr);
   }