Expand cloneLoopWithPreheader() to support cloning loop nest

Summary: cloneLoopWithPreheader() currently only support innermost loop,
and assert otherwise.
Reviewers: Meinersbur, fhahn, kbarton
Reviewed By: Meinersbur
Subscribers: hiraditya, jsji, llvm-commits
Tag: LLVM
Differential Revision: https://reviews.llvm.org/D63446

llvm-svn: 364310