Track execution counts of the continuations of block structures (e.g.
IfStatements) to capture cases in which execution does not continue after a
block. For example:

for (;;) {
  return;
}
// Never reached, tracked by continuation counter.

A continuation counter only has a start position; it's range is implicitly
until the next sibling range or the end of the parent range.

Bug: v8:6000
Change-Id: I8e8f1f5b140b64c86754b916e626eb50f0707d70
Reviewed-on: https://chromium-review.googlesource.com/530846
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46006}