- Use integer log2
- Fix lower bucket index
- Introduce upper bucket index

Bug: v8:7133
Change-Id: I1f1209a13d69ed6e27eeb18254fccb8263f62954
Reviewed-on: https://chromium-review.googlesource.com/793450Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49677}