[llvm] [CycleInfo] Store blocks using Euler tour representation (PR #208614)

[Fangrui Song via llvm-commits]

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@@ -79,13 +82,16 @@ template <typename ContextT> class GenericCycle {
   ///       always have the same depth.
   unsigned Depth = 0;
 
+  /// The cycle info that owns this cycle. Used by contains(BlockT*).
+  const GenericCycleInfo<ContextT> *CI = nullptr;
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MaskRay wrote:

There are many direct and transitive callers

- Direct: blocks() / block_begin() / block_end() (read CI->BlockLayout), contains(const BlockT*) (CI->getCycle).
 - Transitive (call the above internally): getExitBlocks, getExitingBlocks, getCyclePredecessor, getCyclePreheader, print, verifyCycle/verifyCycleNest.

I don't think it's feasible to add the `CI` parameter in this PR.

https://github.com/llvm/llvm-project/pull/208614