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

[Florian Hahn via llvm-commits]

================
@@ -285,23 +331,19 @@ template <> struct GraphTraits<const VPBlockBase *> {
   }
 };
 
-/// Inverse graph traits are not implemented yet.
-/// TODO: Implement a version of VPBlockNonRecursiveTraversalWrapper to traverse
-/// predecessors recursively through regions.
 template <> struct GraphTraits<Inverse<VPBlockBase *>> {
   using NodeRef = VPBlockBase *;
-  using ChildIteratorType = SmallVectorImpl<VPBlockBase *>::iterator;
+  using ChildIteratorType =
+      VPImmediateHierarchicalChildrenIterator<VPBlockBase *, /*Forward=*/false>;
 
-  static NodeRef getEntryNode(Inverse<NodeRef> B) {
-    llvm_unreachable("not implemented");
-  }
+  static NodeRef getEntryNode(Inverse<NodeRef> B) { return B.Graph; }
 
   static inline ChildIteratorType child_begin(NodeRef N) {
-    llvm_unreachable("not implemented");
+    return ChildIteratorType{N};
----------------
fhahn wrote:

If possible, LLVM recommends using () for constructors (https://llvm.org/docs/CodingStandards.html#do-not-use-braced-initializer-lists-to-call-a-constructor)

```suggestion
    return ChildIteratorType(N);
```


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