[turbofan] Initial work on cleaning up the Node class.

- Remove use_count_, only used for debug builds and tests.
- Use BitField instead of the unreliable C++ bit fields.
- Improve memory layout.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/780503002

Cr-Commit-Position: refs/heads/master@{#25632}