[mlir] Add an option to control the number of loops in affine parallelizer

Add a pass option to control the number of nested parallel loops produced by
the parallelization passes. This is useful to build end-to-end passes targeting
systems that don't need multiple parallel dimensions (e.g., CPUs typically need
only one).

Reviewed By: wsmoses, chelini

Differential Revision: https://reviews.llvm.org/D92765