[OpenMP] Use compound operators for reduction combiner if available.

The OpenMP spec seems to require the compound operators be used for
+, *, &, |, and ^ reduction.  So use these if a class has those operators.
If not try the simple operators as we did previously to limit the impact
to existing code.

Fixes: https://bugs.llvm.org/show_bug.cgi?id=48584

Differential Revision: https://reviews.llvm.org/D101941