These instructions were changed from "s8x16" to "i8x16" prefixes in
https://github.com/WebAssembly/simd/pull/321.

This CL updates all V8 code, including arch-specific code, to match.

Bug: v8:10946, v8:10933
Change-Id: I26ef9ad77571f94501d42c1d65f57380fd507f3d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2432068Reviewed-by: Zhi An Ng <zhin@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70143}