Narrow array subscripts correctly, even in the presence of other datatype errors (so we do not generate error messages for array subscripts that do not contain errors).
Things that we must still check for in stage 3:
1) Handling of CONSTANTs:
1.a) "Any program organization unit attempts to modify the value of a variable that has been declared with the CONSTANT qualifier;"
1.b) From table 16.a "The CONSTANT qualifier shall not be used in the declaration of function block instances as described in 2.5.2.1."