Merging r318848:

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r318848 | hahnfeld | 2017-11-22 09:15:20 -0800 (Wed, 22 Nov 2017) | 7 lines

Fix for OMP doacross implementation on Power

Power has a weak consistency model so we need memory barriers to
make writes (both from runtime and from user code) available for
all threads.

Differential Revision: https://reviews.llvm.org/D40175
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llvm-svn: 319057