Add a test case to show isKnownNonZero() returns correctly; NFC

Summary:
Added a test case just to make sure that isKnownNonZero() returns false
when we cannot guarantee that a ConstantExpr is a non-zero constant.

Reviewers: sanjoy, majnemer, mcrosier, nlewycky

Subscribers: nlewycky, mssimpso, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D16908

llvm-svn: 260544