[LoopUnroll] Convert some existing tests to unit-tests.

Summary: As we now have unit-tests for UnrollAnalyzer, we can convert some existing tests to this format. It should make the tests more robust.

Reviewers: chandlerc, sanjoy

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D17904

llvm-svn: 263318