[llvm] [VPlan] Hook IR blocks into VPlan during skeleton creation (NFC) (PR #114292)

[via llvm-commits]

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@@ -621,6 +621,12 @@ class VPBlockBase {
   /// Remove all the successors of this block.
   void clearSuccessors() { Successors.clear(); }
 
+  /// Swap successors of the block. The block must have exactly 2 successors.
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ayalz wrote:

TODO: This should be part of introducing conditional branch recipes rather than being independent.

https://github.com/llvm/llvm-project/pull/114292