[LoopInterchange] Split up interchange.ll test case (NFC).

Summary:
Currently most tests for the loop interchange pass are in
test/Transforms/LoopInterchange/interchange.ll. This patch splits up the
large test file in smaller pieces, which makes debugging test failures
easier.

Reviewers: karthikthecool, blitz.opensource, hfinkel

Reviewed By: hfinkel

Subscribers: hfinkel, mcrosier, mkuper, mzolotukhin, mssimpso, llvm-commits

Differential Revision: https://reviews.llvm.org/D35488

llvm-svn: 308284