This improves the filter deciding whether a checkpoint needs to be
created. We now keep track of whether a node having an observable
side-effect has been created, allowing to elide checkpoint that are
provably effect-dominated by another checkpoint already.

By now the initial graphs contain an increasing amount of nodes marked
with {Operator::kNoWrite}, making this optimization worthwhile.

R=jarin@chromium.org

Change-Id: Ie7ffb67e1ab081ef7aa3017675afbe5f9e7601ab
Reviewed-on: https://chromium-review.googlesource.com/443466Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43256}