[OPENMP][NVPTX]Use __syncwarp() to reconverge the threads.

Summary:
In Cuda 9.0 it is not guaranteed that threads in the warps are
convergent. We need to use __syncwarp() function to reconverge
the threads and to guarantee the memory ordering among threads in the
warps.
This is the first patch to fix the problem with the test
libomptarget/deviceRTLs/nvptx/src/sync.cu on Cuda9+.
This patch just replaces calls to __shfl_sync() function with the call
of __syncwarp() function where we need to reconverge the threads when we
try to modify the value of the parallel level counter.

Reviewers: grokos

Subscribers: guansong, jfb, jdoerfert, caomhin, kkwli0, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D65013

llvm-svn: 369796