As a drive-by, change an if(...) else UNREACHABLE into a CHECK(...).

Change-Id: I6440191c690f36444faa89ac0f7f7dde51ebba3f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2237143Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68369}