Refactor out LoopInfo computation so that it can be used by

other test cases.

Summary: Refactor out LoopInfo computation so that it can be
used by other test cases.

So i am changing this test proactively for later commit, which will use
this function.

Reviewers: sanjoy, hfinkel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D28778

llvm-svn: 292250