This makes sure {NodeProperties::FindFrameStateBefore} can deal with
effect chains that are marked as dead. This can happen when reducers
looking for frame states run together with other reducers killing some
execution paths within the same reduction phase.

R=bmeurer@chromium.org
TEST=mjsunit/regress/regress-crbug-617567
BUG=chromium:617567,chromium:617224

Review-Url: https://codereview.chromium.org/2041833002
Cr-Commit-Position: refs/heads/master@{#36743}