[CloneFunction] Support BB == PredBB in DuplicateInstructionsInSplit.

In case PredBB == BB and StopAt == BB's terminator, StopAt != &*BI will
fail, because BB's terminator instruction gets replaced.

By using BB.getTerminator() we get the current terminator which we can use
to compare.

Reviewers: sanjoy, anna, reames

Reviewed By: anna

Differential Revision: https://reviews.llvm.org/D43822

llvm-svn: 326779