[LoopInterchange] Update code to use range-based for loops (NFC).

Summary:
The remaining non range-based for loops do not iterate over full ranges,
so leave them as they are.

Reviewers: karthikthecool, blitz.opensource, mcrosier, mkuper, aemerson

Reviewed By: aemerson

Subscribers: aemerson, mzolotukhin, llvm-commits

Differential Revision: https://reviews.llvm.org/D35777

llvm-svn: 308872