Cleanup to simulator to remove repetitive logic to get instruction
fields.

Bug: v8:10997
Change-Id: I01f0b99f85788b41e4cab505fc94362d637c396f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2554256
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71391}