[turbofan] Check for dead node in the common operator reducer.

[turbofan] Check for dead node in the common operator reducer.

BUG=chromium:556543
LOG=n

Committed: https://crrev.com/a77f9179e0135afcfe6f62f1451f27517c6b9c61
Cr-Commit-Position: refs/heads/master@{#32025}

[Jarin, 2015-11-16 15:47:31]

jarin@chromium.org changed reviewers:
+ bmeurer@chromium.org

[Jarin, 2015-11-16 15:47:32]

Benedikt, could you take a look?

It looks like during VisitPhi, the IfTrue and IfFalse nodes have Dead input
because we are just propagating 'deadness' there. When we try to take the
condition input from the killed branch, we crash. The simple fix is to check
whether the branch is still alive, but you might have some more general idea on
how to fix this.

[Benedikt Meurer, 2015-11-16 17:40:42]

Well, I think this is the correct fix, because there's no way to be sure about
the exact point in time when DCE kicks in due to GraphReducer stack/revisit
queue mechanism wrt. loops. So this is LGTM.

[Jarin, 2015-11-17 09:01:26]

The CQ bit was checked by jarin@chromium.org

[commit-bot: I haz the power, 2015-11-17 09:01:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1450883003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1450883003/1

[commit-bot: I haz the power, 2015-11-17 09:03:03]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-17 09:03:18]

Patchset 1 (id:??) landed as
https://crrev.com/a77f9179e0135afcfe6f62f1451f27517c6b9c61
Cr-Commit-Position: refs/heads/master@{#32025}