MathExtras UnitTest: Assert that isPowerOf2(0) is false. NFC.

Summary:
This is a follow-up on D34077. Elena observed that the
correctness of the code relies on isPowerOf2(0) returning false.
Adding a test to cover this corner-case.

Reviewers: delena, davide, craig.topper

Reviewed By: davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34939

llvm-svn: 307046