[llvm] [FixIrreducible] Use CycleInfo instead of a custom SCC traversal (PR #101386)

[Jakub Kuderski via llvm-commits]

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@@ -107,6 +107,12 @@ template <typename ContextT> class GenericCycle {
     return is_contained(Entries, Block);
   }
 
+  /// \brief Replace all entries with \p Block as single entry.
+  void setSingleEntry(BlockT *Block) {
+    Entries.clear();
+    Entries.push_back(Block);
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kuhar wrote:

Could this be `nullptr`? Should we add an assert to guard against this?

https://github.com/llvm/llvm-project/pull/101386