[llvm] [VPlan] Simplify the computation of the block entry mask. (PR #173265)

[Mel Chen via llvm-commits]

================
@@ -113,6 +113,74 @@ class VPAllSuccessorsIterator
   }
 };
 
+/// Similar to VPAllSuccessorsIterator but for predecessors. For VPRegionBlocks,
+/// visits their exiting block instead of direct predecessors.
+template <typename BlockPtrTy>
+class VPAllPredecessorsIterator
+    : public iterator_facade_base<VPAllPredecessorsIterator<BlockPtrTy>,
+                                  std::forward_iterator_tag, VPBlockBase> {
+  BlockPtrTy Block;
+  /// Analogous to SuccessorIdx in VPAllSuccessorsIterator but for the index of
+  /// predecessor.
+  size_t PredecessorIdx;
----------------
Mel-Chen wrote:

Thank you! Rebased on your branch :)

https://github.com/llvm/llvm-project/pull/173265