Add a range type to PostOrderAnalysis for use with the post order and reverse post order ranges. This makes it cleaner if one needs to refer to the actual type returned by the post order and reverse post order calls.

Swift SVN r24165

include/swift/SILAnalysis/PostOrderAnalysis.h

@@ -70,6 +70,9 @@ public:
   using iterator = std::vector<SILBasicBlock *>::iterator;
   using reverse_iterator = std::vector<SILBasicBlock *>::reverse_iterator;
 
+  using range = Range<iterator>;
+  using reverse_range = Range<reverse_iterator>;
+
   Range<iterator> getPostOrder(SILFunction *F) {
     FunctionPOTInfo &POTInfo = FunctionToPOTMap[F];
     if (POTInfo.IsInvalidated)