[LoopUnroll] Use addClonedBlockToLoopInfo to clone the top level loop (NFC)

[LoopUnroll] Use addClonedBlockToLoopInfo to clone the top level loop (NFC)

Summary:
rL293124 added the necessary infrastructure to properly add the cloned
top level loop to LoopInfo, which means we do not have to do it manually
in CloneLoopBlocks.

@mkuper sorry for not pointing this out during my review of D29156, I just
realized that today.

Reviewers: mzolotukhin, chandlerc, mkuper

Reviewed By: mkuper

Subscribers: llvm-commits, mkuper

Differential Revision: https://reviews.llvm.org/D29173

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@293615 91177308-0d34-0410-b5e6-96231b3b80d8