[turbofan] Greedy: smarter last resort splitting.

[turbofan] Greedy: smarter last resort splitting.

When all heuristics fail, we run a "last resort" heuristic. Before, it was
splitting at the first found splittable position either before or after a
use position. That turns out to be too naive: it may split in loops, when
alternative split positions exist outside loops.

This change chooses a "before" use case location that is outside the
loop.

Committed: https://crrev.com/d5a9dd6539d49f0fc7bd2d48f4783520a9e3b88d
Cr-Commit-Position: refs/heads/master@{#30990}

[Mircea Trofin, 2015-09-29 00:47:46]

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

[Benedikt Meurer, 2015-09-29 03:43:26]

LGTM

[Mircea Trofin, 2015-09-29 03:45:15]

The CQ bit was checked by mtrofin@chromium.org

[commit-bot: I haz the power, 2015-09-29 03:45:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1372213005/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1372213005/20001

[commit-bot: I haz the power, 2015-09-29 03:47:28]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-09-29 03:47:43]

Patchset 2 (id:??) landed as
https://crrev.com/d5a9dd6539d49f0fc7bd2d48f4783520a9e3b88d
Cr-Commit-Position: refs/heads/master@{#30990}