Array.p.reduce[Right] did not correctly treat holey arrays
when dealing with mixed holey/non-holey receiver maps.

Bug: chromium:804956
Change-Id: Ic22f64a1beeb93005809948299cde1f1d21a0f19
Reviewed-on: https://chromium-review.googlesource.com/883241Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50864}