[LoopDist] Move loop-versioning helper functions to Cloning, NFC

Summary:
This makes them available to the LoopVersioning class as that is moved
to its own module in the next patch.

Reviewers: ashutosh.nema, hfinkel

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10576

llvm-svn: 241931