[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=

Review URL: https://codereview.chromium.org/1052753004

Cr-Commit-Position: refs/heads/master@{#27620}