In LowerStringCodePointAt, rather than call StringCodePointAt
builtin, we could inline it using StringCharCodeAt.

Bug: v8:11743
Change-Id: I924f4180ffcfd583cfcbba57b2e0cf114adef068
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3517935Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79598}