This fixes a crash noticed when doing a downstream merge. The
test case has been reduced, and is included in this commit.
The existing bailout for non-power-of-two vectors in TryToFindDuplicates
did not consider the case where the list being vectorized had no
root node. This allowed reshuffled scalars to slip through to code
which does not yet expect to handle it.
This was an existing bug (likely introduced by my ed03070e), but
made easier to hit by 63e8a1b1
247d3ea843