And removed the ifdef guards around instruction-selector and
tests since v128.const is now implemented for x86, x64, arm, arm64.

Bug: v8:8460
Change-Id: I0ed8aede0a07db2fd286bf0c3385eba1079558f8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2285149
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68745}