Array index resolution: fix use of goto_symex_statet::rename

This used to accidentally discard its argument, losing the possibility to replace an
array index with its true value.

Author: smowton

src/goto-symex/symex_dereference.cpp

@@ -237,8 +237,7 @@ void goto_symext::dereference_rec(exprt &expr, statet &state, bool write)
         const exprt &array_cell = to_index_expr(*expr_it).index();
         if(!array_cell.is_constant())
         {
-          exprt constant_array_cell = array_cell;
-          state.rename(constant_array_cell, ns);
+          exprt constant_array_cell = state.rename(array_cell, ns).get();
           do_simplify(constant_array_cell);
           if(constant_array_cell.is_constant())
           {
@@ -255,8 +254,7 @@ void goto_symext::dereference_rec(exprt &expr, statet &state, bool write)
         const exprt &rhs = expr_it->op1();
         if(!rhs.is_constant())
         {
-          exprt constant_rhs = rhs;
-          state.rename(constant_rhs, ns);
+          exprt constant_rhs = state.rename(rhs, ns).get();
           do_simplify(constant_rhs);
           if(constant_rhs.is_constant())
           {