Optimize insertion sort

Optimize insertion sort

This change slightly changes the main iteration loop so that LLVM can
optimize it more efficiently.

Benchmark:

name                                   before ns/iter   after ns/iter    diff ns/iter   diff %
slice::sort_unstable_small_ascending   39 (2051 MB/s)   38 (2105 MB/s)             -1   -2.56%
slice::sort_unstable_small_big_random  579 (2210 MB/s)  575 (2226 MB/s)            -4   -0.69%
slice::sort_unstable_small_descending  80 (1000 MB/s)   70 (1142 MB/s)            -10  -12.50%
slice::sort_unstable_small_random      396 (202 MB/s)   386                       -10   -2.53%