Minor optimizations in product kernel: - use pbroadcast4 (helpful when AVX is not available) - process all remaining rows at once (significant speedup for small matrices)
Minor optimizations in product kernel:
- use pbroadcast4 (helpful when AVX is not available)
- process all remaining rows at once (significant speedup for small matrices)

madlib/eigen_backup,rotorliu/eigen,rotorliu/eigen,mjbshaw/Eigen,rotorliu/eigen,madlib/eigen_backup,madlib/eigen_backup,mjbshaw/Eigen,mjbshaw/Eigen,madlib/eigen_backup,rotorliu/eigen,mjbshaw/Eigen