This fixes the bug where the reducer ignores a prototype that is not
initial. Tests are also added.

Bug: v8:8056
Change-Id: I428eed2d2790fffa22f67a051f7d1f1e4d3ce947
Reviewed-on: https://chromium-review.googlesource.com/1174542Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Hai Dang <dhai@google.com>
Cr-Commit-Position: refs/heads/master@{#55149}