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

[Ryotaro Kasuga via llvm-commits]

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@@ -497,11 +498,13 @@ struct LoopInterchange {
     // indicates the loop should be placed as the innermost loop.
     //
     // For the old pass manager CacheCost would be null.
-    DenseMap<const Loop *, unsigned> CostMap;
+    std::optional<CostMapTy> CostMap = std::nullopt;
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kasuga-fj wrote:

The `CostMap` in `LoopInterchangeProfitability` is a reference to this object. I just changed the way it is passed: from the argument of `isProfitable` to the ctor.
I think we should clean up the code in this area. For example, the arguments `InnerLoop` and `OuterLoop` of `isProfitable` look redundant. They would be the same as what is passed by the ctor.

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