[turbofan] Fix bug in OSR deconstruction.

[turbofan] Fix bug in OSR deconstruction.

In constructing the transfer between loop copies, we need to merge the backedges from all the previous copies of the given loop. The control reduction will work out which ones are really reachable.

R=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1004993004

Cr-Commit-Position: refs/heads/master@{#27246}