Fix Reduction.inames_set to return frozenset instead of mutable set (#998)

* Initial plan

* Fix Reduction.inames_set to return frozenset instead of mutable set

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Co-authored-by: inducer <352067+inducer@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: inducer <352067+inducer@users.noreply.github.com>

Author: Copilot

loopy/symbolic.py

@@ -931,8 +931,8 @@ def is_tuple_typed(self) -> bool:
         return self.operation.arg_count > 1
 
     @cached_property
-    def inames_set(self) -> set[str]:
-        return set(self.inames)
+    def inames_set(self) -> frozenset[str]:
+        return frozenset(self.inames)
 
 
 @p.expr_dataclass()