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

[Florian Hahn via llvm-commits]

================
@@ -25,90 +25,132 @@ namespace llvm {
 // GraphTraits specializations for VPlan Hierarchical Control-Flow Graphs     //
 //===----------------------------------------------------------------------===//
 
-/// Iterator to traverse all successors of a VPBlockBase node. This includes the
-/// entry node of VPRegionBlocks. Exit blocks of a region implicitly have their
-/// parent region's successors. This ensures all blocks in a region are visited
-/// before any blocks in a successor region when doing a reverse post-order
-// traversal of the graph. Region blocks themselves traverse only their entries
-// directly and not their successors. Those will be traversed when a region's
-// exiting block is traversed
-template <typename BlockPtrTy>
-class VPAllSuccessorsIterator
-    : public iterator_facade_base<VPAllSuccessorsIterator<BlockPtrTy>,
-                                  std::bidirectional_iterator_tag,
-                                  VPBlockBase> {
+/// Iterator to traverse all successors/predecessors of a VPBlockBase node, such
+/// that:
+///
+///     A
+///     |
+///  +-----+ <- Region R
+///  |  b  |
+///  |     |
+///  | ... |
+///  |     |
+///  |  e  |
+///  +-----+
+///     |
+///     B
+///
+///  children(A) == {R} ; Forward == true
+///  children(R) == {b} ; Forward == true
+///  children(e) == {B} ; Forward == true
+///
+///  children(B) == {R} ; Forward == false
+///  children(R) == {e} ; Forward == false
+///  children(b) == {A} ; Forward == false
+///
+/// This ensures that all blocks of the region are visited before continuing
----------------
fhahn wrote:

It looks like this dropped the preceding explanation (`Exit blocks of a region implicitly have their parent region's successors.`)

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