A previous change (see ref) added a subset of 256-bit instructions to
the x64 assembler--this change adds a disassembly test for the added
instructions.

ref: https://chromium-review.googlesource.com/c/v8/v8/+/3123648
Change-Id: Ia56be7a7df636b8bf6c04f044912e914d949d19f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3133956
Auto-Submit: Andrew Brown <andrew.brown@intel.com>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76711}