The unwinding info writer was assuming that a block that had no initial
state must be the start or end block. It was trying to check if the
block was the start or end by asserting that the block had either
no predecessors or no successors. Loop rotation breaks this assumption,
since it can move a block from inside a loop to before the loop
header, and the block can then be the "start" block. But since
such a block has both predecessor(s) and a successor, the check was
wrong.

R=clemensh@chromium.org
BUG=chromium:913844

Change-Id: Ic24c67a822d510cb082f25608089d313c3459be8
Reviewed-on: https://chromium-review.googlesource.com/c/1373770
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58675}