Instead, always pass an index and an enum pointing to either half.

R=titzer@chromium.org

Bug: v8:6600, v8:8562
Change-Id: I7e73bd97bfc7ebf644b242980aa0a73cd5f18949
Reviewed-on: https://chromium-review.googlesource.com/c/1392189
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58516}