[OPENMP][NVPTX]Improve performance + reduce number of used registers.

Summary:
Reduced number of the used register + improved performance propagating
the information about current execution/data sharing mode directly from
the compiler, where it is possible.
In some cases, it requires new/reworked interfaces of the runtime
external functions. Old functions are marked as deprecated.

Reviewers: grokos, gtbercea, kkwli0

Subscribers: guansong, jfb, openmp-commits, caomhin

Differential Revision: https://reviews.llvm.org/D56278

llvm-svn: 350405