ARROW-9699: [C++][Compute] Optimize mode kernel for small integer types

[cyb70289]

For small integers(bool, int8, uint8), instead of general hash table,
using a value indexed array improves performance about 2x ~ 6x.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-9699

[cyb70289]

Removed unrelated tests for clarity.
100% null benchmark is not stable, can be ignored.

$ archery benchmark diff --suite-filter="arrow-compute-aggregate-benchmark" --cc=clang-9 --cxx=clang++-9

                      benchmark         baseline        contender  change % 
   ModeKernelInt8/1048576/10000   60.196 MiB/sec  477.048 MiB/sec   692.487 'null_percent': 0.01
     ModeKernelInt8/1048576/100   60.548 MiB/sec  470.066 MiB/sec   676.351, 'null_percent': 1.0
       ModeKernelInt8/1048576/0   63.520 MiB/sec  472.362 MiB/sec   643.638, 'null_percent': 0.0
      ModeKernelInt8/1048576/10   62.997 MiB/sec  420.059 MiB/sec   566.788 'null_percent': 10.0
   ModeKernelBoolean/1048576/10    7.640 MiB/sec   28.329 MiB/sec   270.793 'null_percent': 10.0
ModeKernelBoolean/1048576/10000    7.332 MiB/sec   26.496 MiB/sec   261.362 'null_percent': 0.01
  ModeKernelBoolean/1048576/100    7.415 MiB/sec   26.710 MiB/sec   260.212, 'null_percent': 1.0
    ModeKernelBoolean/1048576/0    8.079 MiB/sec   28.428 MiB/sec   251.895, 'null_percent': 0.0
       ModeKernelInt8/1048576/2   80.291 MiB/sec  265.673 MiB/sec   230.887 'null_percent': 50.0
    ModeKernelBoolean/1048576/2   12.449 MiB/sec   27.915 MiB/sec   124.235 'null_percent': 50.0
       ModeKernelInt8/1048576/1  864.966 MiB/sec    1.313 GiB/sec    55.455'null_percent': 100.0
    ModeKernelBoolean/1048576/1  115.581 MiB/sec  153.801 MiB/sec    33.067'null_percent': 100.0
  ModeKernelInt32/1048576/10000  228.572 MiB/sec  263.524 MiB/sec    15.292 'null_percent': 0.01
    ModeKernelInt32/1048576/100  230.087 MiB/sec  265.102 MiB/sec    15.218, 'null_percent': 1.0
  ModeKernelInt16/1048576/10000  115.973 MiB/sec  132.465 MiB/sec    14.221 'null_percent': 0.01
    ModeKernelInt16/1048576/100  116.657 MiB/sec  132.673 MiB/sec    13.729, 'null_percent': 1.0
     ModeKernelInt32/1048576/10  241.338 MiB/sec  270.446 MiB/sec    12.061 'null_percent': 10.0
     ModeKernelInt16/1048576/10  122.426 MiB/sec  135.675 MiB/sec    10.822 'null_percent': 10.0
  ModeKernelInt64/1048576/10000  481.018 MiB/sec  523.582 MiB/sec     8.849 'null_percent': 0.01
    ModeKernelInt64/1048576/100  485.800 MiB/sec  527.232 MiB/sec     8.529, 'null_percent': 1.0
     ModeKernelInt64/1048576/10  526.491 MiB/sec  569.412 MiB/sec     8.152 'null_percent': 10.0
      ModeKernelInt64/1048576/1    6.671 GiB/sec    7.180 GiB/sec     7.623'null_percent': 100.0
      ModeKernelInt64/1048576/2  783.253 MiB/sec  813.436 MiB/sec     3.854 'null_percent': 50.0
      ModeKernelInt32/1048576/0  257.627 MiB/sec  265.602 MiB/sec     3.096, 'null_percent': 0.0
      ModeKernelInt16/1048576/0  128.580 MiB/sec  132.340 MiB/sec     2.924, 'null_percent': 0.0
      ModeKernelInt16/1048576/2  160.189 MiB/sec  164.776 MiB/sec     2.863 'null_percent': 50.0
      ModeKernelInt64/1048576/0  514.855 MiB/sec  528.897 MiB/sec     2.727, 'null_percent': 0.0
      ModeKernelInt32/1048576/2  321.334 MiB/sec  324.092 MiB/sec     0.858 'null_percent': 50.0
      ModeKernelInt16/1048576/1    1.860 GiB/sec    1.653 GiB/sec   -11.109'null_percent': 100.0
      ModeKernelInt32/1048576/1    5.728 GiB/sec    3.263 GiB/sec   -43.038'null_percent': 100.0

[pitrou]

Thank you for doing this.
A more sophisticated optimization for integer arrays would be to find the min/max, and use a direct access table if max-min is small enough (e.g. < 4096). But that can be left for another JIRA.

[pitrou]

+1, thank you @cyb70289 !