Move a field and pack enums in Truncation to save 2 word for
each NodeInfo.

BUG=v8:9684

Change-Id: Ib470019b13a1cb8586c1bc585ff1aff6a88267ef
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2892664
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74597}