These conversion instructions were removed from the proposal in
https://github.com/WebAssembly/simd/pull/178.

Change-Id: I212ca2f923362bf08e178f6d28cc2338cf6f5927
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2016006Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66015}