[mlir][flang][openmp] Rework wsloop reduction operations

This patch reworks the way that wsloop reduction operations function to better
match the expected semantics from the OpenMP specification, following the rework
of parallel reductions.

The new semantics create a private reduction variable as a block argument which
should be used normally for all operations on that variable in the region; this
private variable is then combined with the others into the shared variable. This
way no special omp.reduction operations are needed inside the region. These
block arguments follow the loop control block arguments.