Avoid constructing the frame summary (and a std::vector) just for
getting the function index. Just get it from the code instead (where
also the frame summary would get it from).

R=jkummerow@chromium.org

Bug: v8:11074
Change-Id: Ie9957e145d6b641fb211b03ef593d57afd310c91
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2653230Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72427}