The (currently four, soon five) RelocIterator constructors contain
basically identical logic. Refactor that into a basic version that all
other constructors call.

Bug: v8:6666
Change-Id: Ice7b4891d5e539ff6fe63337fc52d480d85dc270
Reviewed-on: https://chromium-review.googlesource.com/1059109
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53189}