[NFC][Flang][OpenMP] Refactor OpenMP.cpp::genOpenMPReduction

This patch serves two main purposes:
Firstly, to split some of the logic into a seperate method
to try and improve readability

On top of this, it aims to make creating the reductions more generic.
That way, subsequent patches adding reductions shouldn't need
to add a significant amount of extra logic checks, such as checking
for specific operators.

Reviewed By: awarzynski

Differential Revision: https://reviews.llvm.org/D131161