[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).

I failed to update the phi nodes properly in the last patch https://reviews.llvm.org/rL300657.

Phi nodes values are per predecessor in LLVM.

Reviewers: sanjoy

Reviewed By: sanjoy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32400

llvm-svn: 301139