Fix canonicalization of >2 loops with the same counter

Previously, when there were more than two loops with the same counter, e.g.

for (int i = 0; false; );
for (int i = 0; false; );
for (int i = 0; false; );

the canonicalization would only duplicate the counter once, resulting in

for (int i__0 = 0; false; );
for (int i__0 = 0; false; );
for (int i = 0; false; );

This MR fixes the issue, so that all symbols are unique:

for (int i__1 = 0; false; );
for (int i__0 = 0; false; );
for (int i = 0; false; );