Elminate BCE in mask algorithm

Thanks again to @renthraysk

This provides another significant speedup.

benchmark                        old MB/s     new MB/s     speedup
Benchmark_mask/2/fast-8          405.48       513.25       1.27x
Benchmark_mask/3/fast-8          518.93       661.92       1.28x
Benchmark_mask/4/fast-8          1207.10      1252.39      1.04x
Benchmark_mask/8/fast-8          1708.82      1655.63      0.97x
Benchmark_mask/16/fast-8         3418.58      3051.25      0.89x
Benchmark_mask/32/fast-8         5789.43      5813.31      1.00x
Benchmark_mask/128/fast-8        12819.53     14804.50     1.15x
Benchmark_mask/512/fast-8        18247.06     21659.50     1.19x
Benchmark_mask/4096/fast-8       19802.31     23885.68     1.21x
Benchmark_mask/16384/fast-8      20896.97     25081.11     1.20x