[OpenMP][NVPTX] Simplify codegen for orphaned parallel, NFCI.

Worker threads fork off to the compiler generated worker function
directly after entering the kernel function. Hence, there is no
need to check whether the current thread is the master if we are
outside of a parallel region (neither SPMD nor parallel_level > 0).

Differential Revision: https://reviews.llvm.org/D52732

llvm-svn: 343618