[OpenMP][Fix] Ensure comparison between unsigned values.

Summary: Ensure the values being compared are both unsigned.

Reviewers: ABataev, Hahnfeld, caomhin, grokos, AndreyChurbanov

Reviewed By: AndreyChurbanov

Subscribers: AndreyChurbanov, guansong, openmp-commits

Differential Revision: https://reviews.llvm.org/D51301

llvm-svn: 340745