Do not use exprt::move_to_operands as it is marked as deprecated

expr.cpp should not be using methods marked as deprecated in its own header
file. This change furthermore increases type safety as the constructor of
`not_exprt` will check that the type of the operand is ID_bool.

Author: tautschnig

src/util/expr.cpp

@@ -104,8 +104,7 @@ void exprt::make_not()
   }
   else
   {
-    new_expr=exprt(ID_not, type());
-    new_expr.move_to_operands(*this);
+    new_expr = not_exprt(*this);
   }
 
   swap(new_expr);