[Cloning] cloneLoopWithPreheader(): add assert to ensure no sub-loops

Summary:
cloneLoopWithPreheader() does not update LoopInfo for sub-loop of
the original loop being cloned. Add assert to ensure no sub-loops for loop being cloned.

Reviewers: anemet, ashutosh.nema, hfinkel

Subscribers: mzolotukhin, llvm-commits

Differential Revision: http://reviews.llvm.org/D15922

llvm-svn: 267671