[mlir] Affine: parallelize affine loops with reductions

Introduce a basic support for parallelizing affine loops with reductions
expressed using iteration arguments. Affine parallelism detector now has a flag
to assume such reductions are parallel. The transformation handles a subset of
parallel reductions that are can be expressed using affine.parallel:
integer/float addition and multiplication. This requires to detect the
reduction operation since affine.parallel only supports a fixed set of
reduction operators.

Reviewed By: chelini, kumasento, bondhugula

Differential Revision: https://reviews.llvm.org/D101171