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GH-18481: [C++] prefer casting literal over casting field ref (#15180)

I ran into this problem while trying to work out partition pruning in the new scan node.  I feel like this is a somewhat naive approach but it seems to work.

I think it would fail if a `DispatchBest` existed where a n-ary kernel existed with non-equal types.  For example, if there was a function foo(int8, int32) and it had a dispatch best of some kind.

Authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: Weston Pace <weston.pace@gmail.com>