[turbofan] Rework handling of loop exits in loop peeling.

[turbofan] Rework handling of loop exits in loop peeling.

This CL primarily makes the loop peeling algorithm more robust; it no longer damages the graph if the loops are improperly closed.

R=bmeurer@chromium.org
BUG=

Committed: https://crrev.com/0b4dfc231263fbe5016f5c2c9e2d2e2fa1c154a1
Cr-Commit-Position: refs/heads/master@{#27620}

[Benedikt Meurer, 2015-04-07 09:32:16]

LGTM

[titzer, 2015-04-07 10:38:40]

The CQ bit was checked by titzer@chromium.org

[commit-bot: I haz the power, 2015-04-07 10:38:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1052753004/1

[commit-bot: I haz the power, 2015-04-07 11:00:46]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-07 11:00:55]

Patchset 1 (id:??) landed as
https://crrev.com/0b4dfc231263fbe5016f5c2c9e2d2e2fa1c154a1
Cr-Commit-Position: refs/heads/master@{#27620}