Also:
 - replace runtime call with cheaper C call
 - use xor instead of addition to remove bias

R=mvstanton@chromium.org

Bug: v8:8212
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: If4bfe0f5fb1864c89f4acd871cb8b74c7cd7ab09
Reviewed-on: https://chromium-review.googlesource.com/1240116
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56165}