[OPNEMP, NVPTX] Fixed sychronization construct + code cleanup.
- Fixed internal problem in __kmpc_barrier function: SPMD mode
synchronization function should be called only in L1 parallel level.
- Removed some extra code for synchronization inside of the code, used
- Some code cleanup.
Reviewers: gtbercea, grokos
Differential Revision: https://reviews.llvm.org/D49564