[mlir][flang][openmp] Rework wsloop reduction operations (#80019)

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.

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Co-authored-by: Kiran Chandramohan <kiran.chandramohan@arm.com>