Load extends always load 64-bits. Previously, we were setting the max
alignment to be the size_log_2 of the load_type. For LoadExtends the
load_type indicates what the lane size to be extended is, *NOT* the size
to be loaded.

Bug: chromium:1082848
Change-Id: I0c4115ea6ec916211b03afdb83376ccc05c0c244
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2202721Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67815}