[OPENMP, NVPTX] Fix loop boundaries calculation for dynamic loops.

Summary:
Patch fixes the next problems.
1. Removes unused functions from omptarget_nvptx_ThreadPrivateContext
class + simplified data members.
2. Fixed calculation of loop boundaries for dynamic loops with static
scheduling.
3. Introduced saving/restoring of the dynamic loop boundaries to support
several nested parallel dynamic loops.

Reviewers: grokos

Subscribers: guansong, kkwli0, openmp-commits

Differential Revision: https://reviews.llvm.org/D49241

llvm-svn: 336915