In the recent port of reduce() and reduceRight(), a check for a length
change during the loop (standard for iterating builtins) was omitted.

We did get array bounds check protection, however it didn't expose
the issue in our tests because the bounds check is against the
backing store length, not against the length in the referring JSArray.

Also added a test for reduceRight().

R=jgruber@chromium.org

Bug: chromium:937676
Change-Id: I76e22e0d71965bff84a0822b1df5dc818a00b50e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1503732Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60033}