[VPlan] Define class successor and predecessor iterators.

[asbirlea]

Summary

Define class-specific successor and predecessor iterators.
These match the existing interface in MachineBasicBlock and clang::CFGBlock.

[asbirlea]

No longer needed. See discussion: http://lists.llvm.org/pipermail/llvm-dev/2020-March/139888.html

llvm/lib/Transforms/Vectorize/VPlan.h

public:
   /// SubclassID field of the VPBlockBase objects. They are used for concrete
   /// type identification.
   using VPBlockTy = enum { VPBasicBlockSC, VPRegionBlockSC };
 
   using VPBlocksTy = SmallVectorImpl<VPBlockBase *>;
 
   virtual ~VPBlockBase() = default;
 
+  using pred_iterator = SmallVectorImpl<VPBlockBase *>::iterator;
+  using const_pred_iterator = SmallVectorImpl<VPBlockBase *>::const_iterator;
+  using succ_iterator = SmallVectorImpl<VPBlockBase *>::iterator;
+  using const_succ_iterator = SmallVectorImpl<VPBlockBase *>::const_iterator;
+  pred_iterator pred_begin() { return Predecessors.begin(); }
+  const_pred_iterator pred_begin() const { return Predecessors.begin(); }
+  pred_iterator pred_end() { return Predecessors.end(); }
+  const_pred_iterator pred_end() const { return Predecessors.end(); }
+  succ_iterator succ_begin() { return Successors.begin(); }
+  const_succ_iterator succ_begin() const { return Successors.begin(); }
+  succ_iterator succ_end() { return Successors.end(); }
+  const_succ_iterator succ_end() const { return Successors.end(); }
+
   const std::string &getName() const { return Name; }
 
   void setName(const Twine &newName) { Name = newName.str(); }
 
   /// \return an ID for the concrete type of this object.
   /// This is used to implement the classof checks. This should not be used
   /// for any other purpose, as the values may change as LLVM evolves.
   unsigned getVPBlockID() const { return SubclassID; }