[loop-unroll] Properly populate LoopInfo for loops cloned in LoopUnrollRuntime.

Summary:
This fixes Transforms/LoopUnroll/runtime-loop3.ll which failed with
EXTENSIVE_DEBUG, because the cloned basic blocks were not added to the
correct sub-loops in LoopUnrollRuntime.cpp.


Reviewers: dexonsmith, mzolotukhin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D28482

llvm-svn: 291619