There is no need to truncate if we are going to extend it again.

At first glance it looks like we can eliminate both steps but
unfortunately the Change is still needed since it will write on the
top bits.

Change-Id: I06d9776384a76f7b2a4454a9176926b3bcef2f2a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2010111Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65886}