This CL implements 4 of the 6 load extend operations. The added
opcodes include: I16x8Load8x8S, I16x8Load8x8U, I32x4Load16x4S,
I32x4Load16x4U.

Bug: v8:9886
Change-Id: I9961f97325168e3a0036e1b282b769cc65b06ffb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1981329
Commit-Queue: Zhiguo Zhou <zhiguo.zhou@intel.com>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Reviewed-by: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65743}