Removing bitfields because they regressed code-load performance on arm.

Bug: v8:7926
Change-Id: Id7f8b26b76203c6a45ba408945abb1a2d1ad9d5b
Reviewed-on: https://chromium-review.googlesource.com/1186419Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Florian Sattler <sattlerf@google.com>
Cr-Commit-Position: refs/heads/master@{#55352}