[llvm] [LoopInterchange] Prevent from undoing its own transformation (PR #127473)

[Michael Kruse via llvm-commits]

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@@ -388,6 +387,11 @@ class LoopInterchangeProfitability {
 
   /// Interface to emit optimization remarks.
   OptimizationRemarkEmitter *ORE;
+
+  /// Map each loop to a pair of (index, cost). The index is the best position
+  /// of this loop, according to CacheCostAnalysis. The cost is the actual cost
+  /// as calculated by CacheCostAnalysis.
+  const std::optional<CostMapTy> &CostMap;
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Meinersbur wrote:

Storing a reference to memory not owned by this object is asking for problems (I am speaking out of experience). If `LoopInterchangeProfitability` cannot own it, it should still be passed when needed.

https://github.com/llvm/llvm-project/pull/127473