This re-arranges the implementation of the function to make it more
consistent. The only functional change is replacement of RoundUp with
RoundDown, which makes more sense for the limits.

Bug: v8:9306
Change-Id: Id1d4bc6cc414e3618c3878de8cb87a9ed59711f5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1643432Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61997}