[JumpThread] We want to fold (not thread) when all predecessor go to single BB's successor. .

Summary: In case all predecessor go to a single successor of current BB. We want to fold (not thread).

Reviewers: efriedma, sanjoy

Reviewed By: sanjoy

Subscribers: dberlin, majnemer, llvm-commits

Differential Revision: https://reviews.llvm.org/D30869

llvm-svn: 300657