[OpenMP][libomptarget] Simplify warp master selection for data sharing

Summary:
There is currently no supported situation where the warp master is not the first thread in the warp.

This also avoids the device execution from hanging on Volta GPUs when ballot_sync is called by a number of threads that is less that the size of a warp.


Reviewers: ABataev, caomhin, grokos

Reviewed By: grokos

Subscribers: guansong, openmp-commits

Differential Revision: https://reviews.llvm.org/D50188

llvm-svn: 342972