[OPENMP][NVPTX]Simplify handling of thread limit, NFC.

Summary:
Patch improves performance of the full runtime mode by moving
threads limit counter to the shared memory. It also allows to save
global memory.

Reviewers: grokos, kkwli0, gtbercea

Subscribers: guansong, jdoerfert, openmp-commits, caomhin

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D61801

llvm-svn: 360584