https://llvm.org/PR48296 shows an example where we delete all of the operands of a phi without actually deleting the phi, and that is invalid IR. The reduced test included here currently crashes for that reason. This was updated relatively recently in D80141.
I'm not familiar with all of the callers and use cases, but it seems there are passes that expect to handle deleting such a phi at a later time. We get regression test failures if we just stub out the DeletePHIIfEmpty argument in removeIncomingValue() for example.
So I changed the logic in BasicBlock::removePredecessor() to allow deletion of the phi if it has only one predecessor to start with because removing the lone predecessor is guaranteed to produce invalid IR in that case. This does not appear to cause problems in other passes.