The load splat, load extend, load zero macros are essentially the same,
consolidate them into a single macro.

Change-Id: Ic812043b37524deb3a9e6ddc223bb95ae77e1d4d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2304715
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68978}