[OPNEMP, NVPTX] Fixed sychronization construct + code cleanup.

Summary:
1. Fixed internal problem in `__kmpc_barrier` function: SPMD mode
synchronization function should be called only in L1 parallel level.
2. Removed some extra code for synchronization inside of the code, used
`__kmpc_barrier` instead.
3. Some code cleanup.

Reviewers: gtbercea, grokos

Subscribers: openmp-commits

Differential Revision: https://reviews.llvm.org/D49564

llvm-svn: 337691