[OPENMP][NVPTX]Save registers for optimized builds with enabled logging.

Summary:
Introduced special noinline function log that allows to save some
registers for optimized builds but with enabled logging. Also, it
increases the stability of the optimized builds with inlined runtime.

Reviewers: gtbercea, kkwli0

Reviewed By: gtbercea

Subscribers: caomhin, guansong, openmp-commits

Differential Revision: https://reviews.llvm.org/D55436

llvm-svn: 348606