[OPENMP][NVPTX]Fix parallel level counter in non-SPMD mode.

Summary:
In non-SPMD mode we may end up with the divergent threads when trying to
increment/decrement parallel level counter. It may lead to incorrect
calculations of the parallel level and wrong results when threads are
divergent. We need to reconverge the threads before trying to modify the
parallel level counter.

Reviewers: grokos, jdoerfert

Subscribers: guansong, openmp-commits, caomhin, kkwli0

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D66802

llvm-svn: 370803