This fixes a few minor issues in the handling of priority top-tier
compilation units, and adds some comment.

1) We document the current design around priority top-tier units.
2) We simplify the code to increase the priority a bit, and make sure to
   avoid integer overflows.
3) We reorder two statements to first increase the
   outstanding_top_tier_functions_ counter before adding a new unit, in
   order to avoid starting to execute a top-tier unit when the counter
   is 0, which would be an inconsistent state.

R=ahaas@chromium.org

Bug: v8:12880
Change-Id: I67bd71135f34b793ea5cf064108668fb72c7e345
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3654097Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80634}