algo: Improve InsSort performance by 2% to 34%.

This change can be reverted when the compiler has improved.

benchmark                   old ns/op    new ns/op    delta
BenchmarkInsSortRand1e2          2601         2407   -7.46%
BenchmarkInsSortRand1e3         82363        79665   -3.28%
BenchmarkInsSortRand1e4       2431472      2380820   -2.08%
BenchmarkInsSortWorst1e2          363          260  -28.37%
BenchmarkInsSortWorst1e3         3271         2161  -33.93%
BenchmarkInsSortWorst1e4        31200        21006  -32.67%