ARROW-15493: [C++][Gandiva] Init ExpressionCacheKey.mode_
ARROW-15493: [C++][Gandiva] Init ExpressionCacheKey.mode_

Class member mode_ is not initialized in one of ExpressionCacheKey
constructors, but it is used to compare equality of two instances.
It causes flaky gandiva-filter-test failures.

Closes #12280 from cyb70289/15493-gandiva-filter

Authored-by: Yibo Cai <e732a3f0de35687e82bda0008a72e443d2f69704@arm.com>
Signed-off-by: Krisztián Szűcs <6bab32dc957a05a87930e779f272342e159326fa@gmail.com>

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