[llvm] [CycleInfo] Reference cycles by preorder index, not pointer. NFC (PR #210271)

[Fangrui Song via llvm-commits]

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@@ -80,12 +80,17 @@ template <typename ContextT> class GenericCycleInfo {
   template <typename> friend class GenericCycleInfoCompute;
 
 private:
+  /// Sentinel for a cycle-index slot that refers to no cycle.
+  static constexpr unsigned NoCycle = ~0u;
+
   /// Internal, data-only storage for a cycle. Consumers name a cycle by a
   /// CycleRef handle and query it through GenericCycleInfo.
   class Cycle {
   public:
-    /// The parent cycle. Is null for top-level cycles.
-    Cycle *ParentCycle = nullptr;
+    /// Preorder index of the parent cycle, or NoCycle for a top-level
+    /// cycle. Before flatten() this holds a creation-order index into
+    /// GenericCycleInfoCompute::AllCycles.
+    unsigned ParentIndex = NoCycle;
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MaskRay wrote:

Follow-up: the dual-phase confusion will be fixed by https://github.com/llvm/llvm-project/pull/210491

https://github.com/llvm/llvm-project/pull/210271