This makes the elimination of checkpoints flowing effect-wise into nodes
having the {Return} operator more permissive. We can cut out checkpoints
even when they are not wholly owned by the return. This also alleviates
a problem where TCO no longer applies.

R=jarin@chromium.org
TEST=mjsunit/regress/regress-crbug-624747
BUG=chromium:624747

Review-Url: https://codereview.chromium.org/2118793002
Cr-Commit-Position: refs/heads/master@{#37480}