[OPENMP, NVPTX] Fixes for NVPTX RTL

Summary:
Patch fixes several problems in the implementation of NVPTX RTL.
1. Detection of the last iteration for loops with static scheduling, no chunks.
2. Fixes reductions for the serialized parallel constructs.
3. Fixes handling of the barriers.

Reviewers: grokos

Reviewed By: grokos

Subscribers: Hahnfeld, guansong, openmp-commits

Differential Revision: https://reviews.llvm.org/D48480

llvm-svn: 335469