Reverts part of the change https://chromium-review.googlesource.com/c/v8/v8/+/2892664
to avoid using bitfields for Truncation since it causes older versions of gcc
to report a warning.

BUG=v8:9684

Change-Id: Ia807d6dbbb5ae1baae73bf5816c336b9b80ae83d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2900660
Auto-Submit: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74624}