We already had an optimization in the CommonOperatorReducer that would
duplicate a Return with Phi, EffectPhi and Merge inputs into the
respective branches. But we can also do the same if the effect input of
the Return dominates all branches, i.e. if the Return and Phi nodes are
the only users of the Merge node.

This helps with the awkward code generation that we currently observe
for || and && in return position.

R=jarin@chromium.org

Review-Url: https://codereview.chromium.org/2668903002
Cr-Commit-Position: refs/heads/master@{#42839}