[OPENMP][NVPTX]Outline assert into noinline function, NFC.

Summary:
At high optimization level asserts lead to some unexpected results
because of auto-inserted unreachable instructions. This outlining
prevents some of such dangerous optimizations and leads to better
stability.

Reviewers: gtbercea, kkwli0, grokos

Subscribers: guansong, caomhin, openmp-commits

Differential Revision: https://reviews.llvm.org/D56101

llvm-svn: 350128