[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

llvm-svn: 293615