1. Use a reverse_range() adaptor for range-based reverse iteration through containers.

2. Remove the emitting of the commented llvm-mc command line.

3. Remove a few TODOs.

4. For commented-out declarations within a class T like this:
   // T(const T&) = delete;
   // T &operator=(const T &) = delete;
Replace them with this:
   T(const T&) = default;
   T &operator=(const T &) = default;
And try to keep them private where possible.

5. Make LivenessNode and TimerTreeNode into internal classes.

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/830303003