TableGen: Change comments about superclass order

[Paul-C-Anagnostopoulos]

Summary

The comments about the SuperClasses list in the Record class says that the list is in reverse pre-order. Unless I've gone mad, the list is actually in post-order. Traversing it backward produces reverse pre-order. I fixed the comments.

[madhur13490]

This comment was removed by madhur13490.

[lattner]

Feel free to directly make comment changes that you think are obvious :)

[Paul-C-Anagnostopoulos]

The patch has been pushed.

llvm/include/llvm/TableGen/Record.h

   SmallVector<Init *, 0> TemplateArgs;
   SmallVector<RecordVal, 0> Values;
 
-  // All superclasses in the inheritance forest in reverse preorder (yes, it
+  // All superclasses in the inheritance forest in post-order (yes, it
   // must be a forest; diamond-shaped inheritance is not allowed).
   SmallVector<std::pair<Record *, SMRange>, 0> SuperClasses;
 

llvm/lib/TableGen/Record.cpp

 
 void Record::getDirectSuperClasses(SmallVectorImpl<Record *> &Classes) const {
   ArrayRef<std::pair<Record *, SMRange>> SCs = getSuperClasses();
+
+  // Superclasses are in post-order, so the final one is a direct
+  // superclass. All of its transitive superclases immediately precede it,
+  // so we can step through the direct superclasses in reverse order.
   while (!SCs.empty()) {
-    // Superclasses are in reverse preorder, so 'back' is a direct superclass,
-    // and its transitive superclasses are directly preceding it.
     Record *SC = SCs.back().first;
     SCs = SCs.drop_back(1 + SC->getSuperClasses().size());
     Classes.push_back(SC);