[OPENMP][NVPTX]Correctly handle L2 parallelism in SPMD mode.

Summary:
The parallelLevel counter must be on per-thread basis to fully support
L2+ parallelism, otherwise we may end up with undefined behavior.
Introduce the parallelLevel on per-warp basis using shared memory. It
allows to avoid the problems with the synchronization and allows fully
support L2+ parallelism in SPMD mode with no runtime.

Reviewers: gtbercea, grokos

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

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D60918

llvm-svn: 359341