[og7] Privatize independent OpenACC reductions

[Cesar Philippidis]

I've applied this patch to openacc-gcc-7-branch which privatizes
reduction variables inside independent loops which are guaranteed to be
assigned worker or vector partitioning during oaccdevlow. Without this
patch, the inner-reduction.c test case would generate bogus results.
This patch is an extension of Chung-Lin's patch posted here
<https://gcc.gnu.org/ml/gcc-patches/2017-09/msg00274.html>.

The next step going forward is to teach the gimplifier to mark those
reduction variables are implicitly firstprivate, instead of
copy+private. Some members in the OpenACC technical committee argue that
the reduction variable in inner-reduction.c should be firstprivate, not
copy.

Cesar
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