[LoopUnroll] Use addClonedBlockToLoopInfo to add loop header to LI (NFC).

Summary:
I have a similar patch up for review already (D29173). If you prefer I
can squash them both together.

Also I think there more potential for code sharing between
LoopUnroll.cpp and LoopUnrollRuntime.cpp. Do you think patches for
that would be worthwhile? 

Reviewers: mkuper, mzolotukhin

Reviewed By: mkuper, mzolotukhin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29311

llvm-svn: 293758