[mlir] AsyncParallelFor: align block size to be a multiple of inner loops iterations

Depends On D115263

By aligning block size to inner loop iterations parallel_compute_fn LLVM can later unroll and vectorize some of the inner loops with small number of trip counts. Up to 2x speedup in multiple benchmarks.

Reviewed By: bkramer

Differential Revision: https://reviews.llvm.org/D115436