[OPENMP][NVPTX]Fix critical region codegen.

Summary:
Previously critical regions were emitted with the barrier making it a
worksharing construct though it is not. Also, it leads to incorrect
behavior in Cuda9+. Patch fixes this problem.

Reviewers: ABataev, jdoerfert

Subscribers: jholewinski, guansong, cfe-commits, grokos

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66673

llvm-svn: 369946