[turbofan] Fix UnifyReturn magic in the inliner.

[turbofan] Fix UnifyReturn magic in the inliner.

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

Committed: https://crrev.com/1dbe83d9fbff359a0fda2c39dcf892989c72ab63
Cr-Commit-Position: refs/heads/master@{#28550}

[Benedikt Meurer, 2015-05-21 12:29:58]

Hey Michi,

As discussed, step 1 of N to fix inlining wrt. Throw, Terminate and Deoptimize.
Not very nice yet, but will be a lot nicer soon(ish). Please take a look.

Thanks,
Benedikt

[Michael Starzinger, 2015-05-21 12:34:19]

LGTM.

[Benedikt Meurer, 2015-05-21 12:34:33]

The CQ bit was checked by bmeurer@chromium.org

[commit-bot: I haz the power, 2015-05-21 12:34:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1146393002/1

[commit-bot: I haz the power, 2015-05-21 13:02:24]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-21 13:02:34]

Patchset 1 (id:??) landed as
https://crrev.com/1dbe83d9fbff359a0fda2c39dcf892989c72ab63
Cr-Commit-Position: refs/heads/master@{#28550}