[PostOrderIterator] Use SmallVector to store stack; NFC

We use a SmallPtrSet to track visited nodes, use a SmallVector
of the same size for the stack.

llvm/include/llvm/ADT/PostOrderIterator.h

@@ -18,6 +18,7 @@
 #include "llvm/ADT/GraphTraits.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/iterator_range.h"
 #include <iterator>
 #include <set>
@@ -101,7 +102,7 @@ class po_iterator
 
   // VisitStack - Used to maintain the ordering.  Top = current block
   // First element is basic block pointer, second is the 'next child' to visit
-  std::vector<std::pair<NodeRef, ChildItTy>> VisitStack;
+  SmallVector<std::pair<NodeRef, ChildItTy>, 8> VisitStack;
 
   po_iterator(NodeRef BB) {
     this->insertEdge(Optional<NodeRef>(), BB);