Add and use Type::subtypes. NFC.

llvm-svn: 222682
diff --git a/llvm/lib/Bitcode/Writer/ValueEnumerator.cpp b/llvm/lib/Bitcode/Writer/ValueEnumerator.cpp
index f065c83..6971d3a 100644
--- a/llvm/lib/Bitcode/Writer/ValueEnumerator.cpp
+++ b/llvm/lib/Bitcode/Writer/ValueEnumerator.cpp
@@ -620,9 +620,8 @@
 
   // Enumerate all of the subtypes before we enumerate this type.  This ensures
   // that the type will be enumerated in an order that can be directly built.
-  for (Type::subtype_iterator I = Ty->subtype_begin(), E = Ty->subtype_end();
-       I != E; ++I)
-    EnumerateType(*I);
+  for (Type *SubTy : Ty->subtypes())
+    EnumerateType(SubTy);
 
   // Refresh the TypeID pointer in case the table rehashed.
   TypeID = &TypeMap[Ty];