[OPENMP][NVPTX]Improved lock/critical constructs.

Summary: Improved support for critical constructs + omp_..._lock... constructs.

Reviewers: gtbercea, kkwli0, caomhin

Subscribers: guansong, jfb, openmp-commits

Differential Revision: https://reviews.llvm.org/D54766

llvm-svn: 347342