ValueMapper/Enumerator: Clean up code in post-order traversals, NFC

ValueMapper/Enumerator: Clean up code in post-order traversals, NFC

Re-layer the functions in the new (i.e., newly correct) post-order
traversals in ValueEnumerator (r266947) and ValueMapper (r266949).
Instead of adding a node to the worklist in a helper function and
returning a flag to say what happened, return the node itself.  This
makes the code way cleaner: the worklist is local to the main function,
there is no flag for an early loop exit (since we can cleanly bury the
loop), and it's perfectly clear when pointers into the worklist might be
invalidated.

I'm fixing both algorithms in the same commit to avoid repeating the
commit message; if you take the time to understand one the other should
be easy.  The diff itself isn't entirely obvious since the traversals
have some noise (i.e., things to do), but here's the high-level change:

    auto helper = [&WL](T *Op) {     auto helper = [](T **&I, T **E) {
                                 =>    while (I != E) {
      if (shouldVisit(Op)) {             T *Op = *I++;
        WL.push(Op, Op->begin());        if (shouldVisit(Op)) {
        return true;                       return Op;
      }                                }
      return false;                    return nullptr;
    };                               };
                                 =>
    WL.push(S, S->begin());          WL.push(S, S->begin());
    while (!empty()) {               while (!empty()) {
      auto *N = WL.top().N;            auto *N = WL.top().N;
      auto *&I = WL.top().I;           auto *&I = WL.top().I;
      bool DidChange = false;
      while (I != N->end())
        if (helper(*I++)) {      =>    if (T *Op = helper(I, N->end()) {
          DidChange = true;              WL.push(Op, Op->begin());
          break;                         continue;
        }                              }
      if (DidChange)
        continue;

      POT.push(WL.pop());        =>    POT.push(WL.pop());
    }                                }

Thanks to Mehdi for helping me find a better way to layer this.

llvm-svn: 267099