[C++] Simplify Call("case_when", ...) expressions

[asfimport]

case_when will be implemented as case_when(struct(bool...), T...) which requires materialization of scalar arguments. However, we can simplify such cases before calling instead, e.g. all scalar false branches can be eliminated, and scalar true branches can be removed and their values promoted to the 'else' argument.

Reporter: David Li / @lidavidm

_{Note: This issue was originally created as ARROW-13325. Please see the migration documentation for further details.}

[github-actions]

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