Load splat opcodes are currently multi-byte, but were not passing the
right lengths for decoding of immediates.

Bug: v8:10258
Change-Id: I2c93c3f915eaa43a74722cf0285f161d16ef0ff6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2154769Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67326}