Improve performance of #pragma omp single

EPCC Performance of single is considerably worse than plain barrier.
Adding a read-only check to the code before the atomic compare-and-store
helps considerably.

Patch by Terry Wilmarth.

Differential Revision: http://reviews.llvm.org/D21893


git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@274369 91177308-0d34-0410-b5e6-96231b3b80d8