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ARROW-8129: [C++][Compute] Refine compare sort kernel

Sorting kernel implements two comparison functions, CompareValues uses
array.Value() for numerical data and CompareViews uses array.GetView()
for non-numerical ones. It can be simplified by using GetView() only
as all data types support GetView(). This patch also refines unit test.

This change improves about 40% performance.

Closes #6640 from cyb70289/sort-refine

Authored-by: Yibo Cai <yibo.cai@arm.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>