When setting a range for a newly allocated lab, consider adjacent cards.
If either is young, don't mark it as kMixed.

Bug: chromium:1029379
Change-Id: If7d1d920dd5769679de68800eae61f3a8dc1eb17
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3584116Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80175}