StringAdd_ConvertRight and StringAdd_ConvertLeft

Change-Id: I7d4c560dd53e445ad73f374824ec6fddcce6a641
Bug: v8:8996
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1756853
Commit-Queue: Z Nguyen-Huu <duongn@microsoft.com>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63381}