[DFAJumpThreading] Handle circular determinator (#78177)

Fixes the buildbot failure in
https://github.com/llvm/llvm-project/pull/78134#issuecomment-1892195197
When we meet the path with single `determinator`, the determinator
actually takes itself as a predecessor. Thus, we need to let `Prev` be
the determinator when `PathBBs` has only one element.