Better handling of splits with factor 1
Better handling of splits with factor 1

These are used to create new trivial loop levels for GPU single-thread
scheduling

jiawen/Halide,jiawen/Halide,psuriana/Halide,ronen/Halide,tdenniston/Halide,ronen/Halide,ronen/Halide,ronen/Halide,kgnk/Halide,ronen/Halide,tdenniston/Halide,jiawen/Halide,psuriana/Halide,psuriana/Halide,kgnk/Halide,kgnk/Halide,kgnk/Halide,jiawen/Halide,ronen/Halide,psuriana/Halide,tdenniston/Halide,tdenniston/Halide,ronen/Halide,psuriana/Halide,jiawen/Halide,jiawen/Halide,tdenniston/Halide,psuriana/Halide,psuriana/Halide,kgnk/Halide,kgnk/Halide,kgnk/Halide,tdenniston/Halide,ronen/Halide,tdenniston/Halide,jiawen/Halide,tdenniston/Halide,kgnk/Halide