Fix for crash in task dependencies

With single thread using __kmpc_omp_wait_deps segfaults in OpenMP runtime.
Offloading with depend also encounters this problem when we generate
kmpc_omp_wait_deps instead of kmpc_omp_task_with_deps.

Patch by Alex Duran

Differential Revision: http://reviews.llvm.org/D21384

llvm-svn: 272949