[Flang][OpenMP] Process reduction only once for parallel do

[kiranchandramohan]

No description provided.

[skatrak]

LGTM. Thank you for catching this, I didn't realize there were exceptions in the standard as to which leaf constructs in a combined construct should get the reduction clause applied to. It says the following (v5.2, Sect. 17.2):

The effect of the reduction clause is as if it is applied to all leaf constructs that permit the clause, except for the following constructs:

• The parallel construct, when combined with the sections, worksharing-loop, loop, or taskloop construct [...]

So I guess we can later refine the check a bit to only avoid processing the reduction clause in these specific cases. But for now I think this is good enough.