Expand `rex_call_operands` function to include when_then_else support

[jdye64]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
Expand the existing rex_call_operands to handle a Expr::Case when_then_else which is currently ignored.

Describe the solution you'd like
Add logic for including when_then_else in the list of operands

Describe alternatives you've considered
None

Additional context
None