Some opcodes are introduced in V8 for prototyping, and performance
measurements that are not officially a part of the current SIMD proposal
but may be included in future, gate these by a separate flag.

Change-Id: Icc6a9e89c6196c8ff144d2e0193d707e1f60c38b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2079539
Commit-Queue: Deepti Gandluri <gdeepti@chromium.org>
Reviewed-by: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Ben Smith <binji@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66542}