use zero_extend_exprt in SMT2 front-end

This replaces the use of a typecast expression by zero_extend_exprt when
parsing an SMT-LIB2 zero_extend expression.

Author: kroening

src/solvers/smt2/smt2_parser.cpp

@@ -693,9 +693,9 @@ exprt smt2_parsert::function_application()
           else if(id=="zero_extend")
           {
             auto width=to_unsignedbv_type(op[0].type()).get_width();
-            unsignedbv_typet unsigned_type(width+index);
+            unsignedbv_typet unsigned_type{width + index};
 
-            return typecast_exprt(op[0], unsigned_type);
+            return zero_extend_exprt{op[0], unsigned_type};
           }
           else if(id == ID_repeat)
           {