[CloneFunction] Preserve DT in DuplicateInstructionsInSplitBetween.

DuplicateInstructionsInSplitBetween can preserve the DT by passing
through DT to SplitEdge.

Reviewers: sanjoy, junbuml, anna, kuhar

Reviewed By: kuhar

Differential Revision: https://reviews.llvm.org/D44629

llvm-svn: 328189