The inliner previously assumed that there will only be returns reaching
the end node, but that's not true.  This refactoring will make it
possible to also hook up Deoptimize, Throw and Terminate nodes reaching
end properly.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/1146393002

Cr-Commit-Position: refs/heads/master@{#28550}