Avoid scatter operation in ExpressionOrExpression case evaluation method (#18444)

## Which issue does this PR close?

- Part of #18075.

## Rationale for this change

The `ExpressionOrExpression` case evaluation method currently uses `zip`
to combine the `then` and `else` results for a batch. This requires a
scatter operation to ensure the partial results are correctly lined up
for the `zip` algorithm.

By using a custom `merge` algorithm, this scatter step can be avoided.

## What changes are included in this PR?

- Introduce a zip variant that does not require prealigning truthy and
falsy result values with the mask array

## Are these changes tested?

Covered by existing case tests

## Are there any user-facing changes?

No

Author: pepijnve