Address a TODO: Use EmitIdentity instead of kArchNop.

Bug: v8:7947
Change-Id: Idd7de1bcffaf56f7eaf5d65be4dae3257d085ea8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1674031Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62338}