[OPENMP][NVPTX]Fix incompatibility of __syncthreads with LLVM, NFC.

Summary:
One of the LLVM optimizations, split critical edges, also clones tail
instructions. This is a dangerous operation for __syncthreads()
functions and this transformation leads to undefined behavior or
incorrect results. Patch fixes this problem by replacing __syncthreads()
function with the assembler instruction, which cost is too high and
wich cannot be copied.

Reviewers: grokos, gtbercea, kkwli0

Subscribers: guansong, openmp-commits, caomhin

Differential Revision: https://reviews.llvm.org/D56274

llvm-svn: 350333