[llvm] [LoopInfo] Add recompute() and use it in SimpleLoopUnswitch (PR #213514)

[Fangrui Song via llvm-commits]

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@@ -854,6 +862,20 @@ template <class BlockT, class LoopT> class LoopInfoBase {
   void analyze(const DominatorTreeBase<BlockT, false> &DomTree);
   ///@}
 
+  /// Rebuild the loop forest from the CFG, reusing the existing loop object for
+  /// every block that still heads a loop. Analyses that key on loop pointers,
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MaskRay wrote:

 Agreed it's worth pursuing separately. `Loop *` is also the loop pass manager's currency (`run(Loop &L, ...)`, LPMUpdater, LAM cache keys), so it's a wide change.

https://github.com/llvm/llvm-project/pull/213514