In order to know which labels are valid continue targets, we must
track the labels that immediately prefix an iteration statement.

Also document some things that I had to figure out.

Bug: v8:8033
Change-Id: Ia8288fd0e553a547aa0f9d1b4381bb103325bc3a
Reviewed-on: https://chromium-review.googlesource.com/1172292Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55110}