32-bit has a 7-bit inline unowned refcount, then 31 bits in the side table. Overflowing the inline count in deinit on an object that didn't already have a side table would crash, because the code assumed that creating a side table in deinit was not allowed.
(64-bit has 31 bits inline and in the side table. Overflowing the inline count immediately overflows the side table as well, so there's no change in behavior there.)
rdar://problem/33765960
