cql3: statement_restrictions: replace has_slice with predicate is_slice check

In the clustering prefix construction loop, replace the has_slice()
call (which uses find_binop to search the merged predicate's expression
tree for slice operators) with a direct check on the individual
predicate vector's is_slice field.

cql3/restrictions/statement_restrictions.cc

@@ -1084,7 +1084,7 @@ statement_restrictions::statement_restrictions(private_tag,
                     break;
                 }
                 prefix.push_back(found->second);
-                if (has_slice(found->second.filter)) {
+                if (std::ranges::any_of(preds, [](const predicate& p) { return p.is_slice; })) {
                     break;
                 }
             }