This adds block coverage support for simple iteration. For-of and
for-in loops are not yet covered, and we don't yet keep execution counts
for init, cond, and next statements.

BUG=v8:6000

Change-Id: I30b468a2c93f0bb60e857b6632be92920f6857e0
Reviewed-on: https://chromium-review.googlesource.com/527113
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45779}