64x2 nodes were already supported, we just didn't add the case for
64x2 load splat. Add that in and we pass simd_load_splat.js.

Bug: v8:10507
Change-Id: If9145e1b3a4595f4707ca0fc3dcee466773c054f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2432074Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70149}