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; );

src/pystencils/backend/transformations/canonicalize_symbols.py

@@ -102,10 +102,11 @@ class CanonicalizeSymbols:
                     cc.mark_as_updated(lhs.symbol)
 
             case PsLoop(ctr, _, _, _, _):
+                decl_symb = ctr.symbol
                 for c in node.children[::-1]:
                     self.visit(c, cc)
                 cc.mark_as_updated(ctr.symbol)
-                cc.end_lifespan(ctr.symbol)
+                cc.end_lifespan(decl_symb)
 
             case PsConditional(cond, then, els):
                 if els is not None: