Fix(optimizer): don't propagate equality constraints from IF/CASE outwards

[georgesittas]

This PR adds support for a prune callback argument in walk_in_scope, similar to how walk works. It then leverages it to fix the bug demonstrated below:

Before:

>>> from sqlglot import parse_one
>>> from sqlglot.optimizer.simplify import simplify
>>>
>>> simplify(parse_one("x = 1 and case when x = 5 then false else true end"), constant_propagation=True).sql()
'FALSE'

After:

>>> from sqlglot import parse_one
>>> from sqlglot.optimizer.simplify import simplify
>>>
>>> simplify(parse_one("x = 1 and case when x = 5 then false else true end"), constant_propagation=True).sql()
'x = 1 AND CASE WHEN FALSE THEN FALSE ELSE TRUE END'

One obvious improvement that can be made in a followup PR (I plan to do this soon) is to reduce conditional expressions such as IF and CASE when the conditions are statically known to be TRUE or FALSE.