[C++][Compute] Optimize variance kernel for integers

[asfimport]

Current variance kernel converts all data type to double before calculation. It's sub-optimal for integers. Integer arithmetic is much faster than floating points, e.g., summation is 4x faster [1].

A quick test for calculating int32 variance shows up to 3x performance gain. Another benefit is that integer arithmetic is accurate.

[1] https://quick-bench.com/q/_Sz-Peq1MNWYwZYrTtQDx3GI7lQ

Reporter: Yibo Cai / @cyb70289
Assignee: Yibo Cai / @cyb70289

PRs and other links:

• GitHub Pull Request #8466

_{Note: This issue was originally created as ARROW-10304. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
For the record, the slowdown seems mostly due to int->double conversion. That doesn't change the overall result, though :-)

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 8466
#8466