Fix wrong code generation if EN/ENO are used in FBD/LD/SFC

This problem appears for example here

--------
------- | MOVE |
| MOVE1 |------|EN ENO|
------- | |
| |
------- | | -----------
| 23 |------|IN OUT|---| LocalVar0 |
------- -------- -----------


--------
------- | MOVE |
| MOVE2 |------|EN ENO|
------- | |
| |
------- | | -----------
| 15 |------|IN OUT|---| LocalVar0 |
------- -------- -----------

Before wrong code was generated for this case:

MOVE6_OUT := MOVE(EN := move1, IN := 23, ENO => MOVE6_ENO);
LocalVar0 := MOVE6_OUT;
MOVE4_OUT := MOVE(EN := move2, IN := 15, ENO => MOVE4_ENO);
LocalVar0 := MOVE4_OUT;

With this patch now following code is generated:

MOVE6_OUT := MOVE(EN := move1, IN := 23, ENO => MOVE6_ENO);
IF MOVE6_ENO THEN
LocalVar0 := MOVE6_OUT;
END_IF;
MOVE4_OUT := MOVE(EN := move2, IN := 15, ENO => MOVE4_ENO);
IF MOVE4_ENO THEN
LocalVar0 := MOVE4_OUT;
END_IF;


See discussion here:
https://sourceforge.net/p/beremiz/mailman/message/36378805/

#!/usr/bin/env python
# -*- coding: utf-8 -*-

# This file is part of Beremiz, a Integrated Development Environment for
# programming IEC 61131-3 automates supporting plcopen standard and CanFestival.
#
# Copyright (c) 2016 Mario de Sousa (msousa@fe.up.pt)
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.
#
# This code is made available on the understanding that it will not be
# used in safety-critical situations without a full and competent review.