The assertion gets exposed when changing the exploration order.
This is a quick hacky fix, but the intention is that if the nodes do
merge, it should not matter which predecessor should be traverse.
A proper fix would be not to traverse predecessors at all, as all
information relevant for any decision should be avilable locally.
rdar://37540480
Differential Revision: https://reviews.llvm.org/D42773
llvm-svn: 325977
George Karpenkov