reduceConditionals should not attempt to reduce variable refs outside of the condition expression

[chrispcampbell]

The reduceConditionals function in the new parse package is supposed to only reduce the condition expression (i.e., the first argument) in an IF THEN ELSE call, but the current implementation is incorrect and tries to reduce any variable-ref expression. We should fix this so that variable-ref expressions are returned unmodified, and only in the case of the condition expression should we attempt to resolve/reduce them to a constant (using reduceExpr).

I will include a test with the fix to make it more clear what the issue is.