Fixes https://bugs.llvm.org/show_bug.cgi?id=44922 (caused by 4698bf145d)
ThreadThroughTwoBasicBlocks assumes PredBBBranch is conditional. The following code can segfault.
AddPHINodeEntriesForMappedBlock(PredBBBranch->getSuccessor(1), PredBB, NewBB,
ValueMapping);
We can also allow unconditional PredBB, but the produced code is not
better.
Reviewed By: kazu
Differential Revision: https://reviews.llvm.org/D74747