[llvm] [VPlan] Generalize `VPAllSuccessorsIterator` to support predecessors (PR #178724)

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``````````bash
git-clang-format --diff origin/main HEAD --extensions cpp,h -- llvm/lib/Transforms/Vectorize/VPlanCFG.h llvm/unittests/Transforms/Vectorize/VPlanTest.cpp --diff_from_common_commit
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``````````diff
diff --git a/llvm/lib/Transforms/Vectorize/VPlanCFG.h b/llvm/lib/Transforms/Vectorize/VPlanCFG.h
index a48ae4841..d9af3b9db 100644
--- a/llvm/lib/Transforms/Vectorize/VPlanCFG.h
+++ b/llvm/lib/Transforms/Vectorize/VPlanCFG.h
@@ -25,8 +25,8 @@ namespace llvm {
 // GraphTraits specializations for VPlan Hierarchical Control-Flow Graphs     //
 //===----------------------------------------------------------------------===//
 
-/// Iterator to traverse all successors/predecessors of a VPBlockBase node, including its hierarchical successors/predecessors. For example:
-/// that:
+/// Iterator to traverse all successors/predecessors of a VPBlockBase node,
+/// including its hierarchical successors/predecessors. For example: that:
 ///
 ///     A
 ///     |

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https://github.com/llvm/llvm-project/pull/178724