[pstl] A hot fix for a reduction parallel pattern of OpenMP backend

Reviewed By: nadiasvertex, ldionne

Differential Revision: https://reviews.llvm.org/D112125

pstl/include/pstl/internal/omp/parallel_reduce.h

@@ -23,6 +23,11 @@ _Value
 __parallel_reduce_body(_RandomAccessIterator __first, _RandomAccessIterator __last, _Value __identity,
                        _RealBody __real_body, _Reduction __reduce)
 {
+    if (__should_run_serial(__first, __last))
+    {
+        return __real_body(__first, __last, __identity);
+    }
+
     auto __middle = __first + ((__last - __first) / 2);
     _Value __v1(__identity), __v2(__identity);
     __parallel_invoke_body(