[PATCH] [OPENMP] Codegen for 'reduction' clause in 'sections' directive.

[Alexey Bataev]

Hi rjmccall, hfinkel,

Emit a code for reduction clause. Next code should be emitted for reductions:
```
static kmp_critical_name lock = { 0 };

void reduce_func(void *lhs[<n>], void *rhs[<n>]) {
  *(Type0*)lhs[0] = ReductionOperation0(*(Type0*)lhs[0], *(Type0*)rhs[0]);
  ...
  *(Type<n>-1*)lhs[<n>-1] =
  ReductionOperation<n>-1(*(Type<n>-1*)lhs[<n>-1],
  *(Type<n>-1*)rhs[<n>-1]);
}

 ...
 void *RedList[<n>] = {&<RHSExprs>[0], ..., &<RHSExprs>[<n>-1]};
 switch (__kmpc_reduce{_nowait}(<loc>, <gtid>, <n>, sizeof(RedList), RedList, reduce_func, &<lock>)) {
 case 1:
  <LHSExprs>[0] = ReductionOperation0(*<LHSExprs>[0], *<RHSExprs>[0]);
  ...
  <LHSExprs>[<n>-1] = ReductionOperation<n>-1(*<LHSExprs>[<n>-1], *<RHSExprs>[<n>-1]);
 __kmpc_end_reduce{_nowait}(<loc>, <gtid>, &<lock>);
 break;
 case 2:
  Atomic(<LHSExprs>[0] = ReductionOperation0(*<LHSExprs>[0], *<RHSExprs>[0]));
  ...
  Atomic(<LHSExprs>[<n>-1] = ReductionOperation<n>-1(*<LHSExprs>[<n>-1], *<RHSExprs>[<n>-1]));
 break;
 default:;
 }
```
Reduction variables are a kind of a private variables, they have private copies, but initial values are chosen in accordance with the reduction operation.
If sections directive has only single section, then original shared variables are used instead with barrier at the end of the directive.

http://reviews.llvm.org/D9242

Files:
  lib/CodeGen/CGStmtOpenMP.cpp
  test/OpenMP/sections_reduction_codegen.cpp

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