[LoopUnroll] Use addClonedBlockToLoopInfo to add loop header to LI (NFC).
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
Differential Revision: https://reviews.llvm.org/D29311