[FixIrreducible] Use CycleInfo instead of a custom SCC traversal

1. CycleInfo efficiently locates all cycles in a single pass, while the SCC is
   repeated inside every natural loop.

2. CycleInfo provides a hierarchy of irreducible cycles, and the new
   implementation transforms each cycle in this hierarchy separately instead of
   reducing an entire irreducible SCC in a single step. This reduces the number
   of control-flow paths that pass through the header of each newly created
   loop. This is evidenced by the reduced number of predecessors on the "guard"
   blocks in the lit tests, and fewer operands on the corresponding PHI nodes.

3. When an entry of an irreducible cycle is the header of a child natural loop,
   the original impleementation destroyed that loop. This is now preserved,
   since the incoming edges on non-header entries are not touched.

Author: ssahasra

llvm/include/llvm/ADT/GenericCycleInfo.h

@@ -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);
+  }
+
   /// \brief Return whether \p Block is contained in the cycle.
   bool contains(const BlockT *Block) const { return Blocks.contains(Block); }
 
@@ -189,6 +195,21 @@ template <typename ContextT> class GenericCycle {
   //@{
   using const_entry_iterator =
       typename SmallVectorImpl<BlockT *>::const_iterator;
+  const_entry_iterator entry_begin() const {
+    return const_entry_iterator{Entries.begin()};
+  }
+  const_entry_iterator entry_end() const {
+    return const_entry_iterator{Entries.end()};
+  }
+
+  using const_reverse_entry_iterator =
+      typename SmallVectorImpl<BlockT *>::const_reverse_iterator;
+  const_reverse_entry_iterator entry_rbegin() const {
+    return const_reverse_entry_iterator{Entries.rbegin()};
+  }
+  const_reverse_entry_iterator entry_rend() const {
+    return const_reverse_entry_iterator{Entries.rend()};
+  }
 
   size_t getNumEntries() const { return Entries.size(); }
   iterator_range<const_entry_iterator> entries() const {
@@ -252,12 +273,6 @@ template <typename ContextT> class GenericCycleInfo {
   /// the subtree.
   void moveTopLevelCycleToNewParent(CycleT *NewParent, CycleT *Child);
 
-  /// Assumes that \p Cycle is the innermost cycle containing \p Block.
-  /// \p Block will be appended to \p Cycle and all of its parent cycles.
-  /// \p Block will be added to BlockMap with \p Cycle and
-  /// BlockMapTopLevel with \p Cycle's top level parent cycle.
-  void addBlockToCycle(BlockT *Block, CycleT *Cycle);
-
 public:
   GenericCycleInfo() = default;
   GenericCycleInfo(GenericCycleInfo &&) = default;
@@ -275,6 +290,12 @@ template <typename ContextT> class GenericCycleInfo {
   unsigned getCycleDepth(const BlockT *Block) const;
   CycleT *getTopLevelParentCycle(BlockT *Block);
 
+  /// Assumes that \p Cycle is the innermost cycle containing \p Block.
+  /// \p Block will be appended to \p Cycle and all of its parent cycles.
+  /// \p Block will be added to BlockMap with \p Cycle and
+  /// BlockMapTopLevel with \p Cycle's top level parent cycle.
+  void addBlockToCycle(BlockT *Block, CycleT *Cycle);
+
   /// Methods for debug and self-test.
   //@{
 #ifndef NDEBUG