We need to trim the graph before we execute the MemoryOptimizer, because
that just walks the effect chain from Start to End and cannot deal with
dead nodes in the use lists.

R=jarin@chromium.org
BUG=chromium:614292

Review-Url: https://codereview.chromium.org/2080703003
Cr-Commit-Position: refs/heads/master@{#37133}