Zero top lanes to ensure that we convert the low lanes of i32x4.

Bug: v8:11265
Change-Id: Id53b65bada406c967c692b584210d4168e2c9183
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2776511Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73587}