PLCgen Siemens target: location variables get assigned non-existing values.

button_lamp example for S7-1200:

            IF syncAutEdge__1 = 1 THEN
                sup := someConstantvariable;
            ELSIF syncAutEdge__1 = 2 THEN
                sup := someConstantvariable;
            END_IF;

while plcopen generates lines like

        IF syncAutEdge__1 = 1 THEN
            sup := sup_s6;
        ELSIF syncAutEdge__1 = 2 THEN
            sup := sup_s7;

Addresses #679