topdown: Fix for unsafe negated query in partial results

In copy propagation we ran into issues where an expression was negated
and required a variable in it to show up in a non-negated expression
to be safe.

What we do now is expose some of the safety helpers from the ast
package and check if any of the removed expressions provided safety
to any vars left in the result (that are otherwise unsafe). If they
do we will re-add the expression.

This isn't done until after computing the initial result to ensure
that we get the minimal set of expressions. If we attempt to check
eagerly, before we kill/remove any, we might let one remain only to
later add an expression to the result that makes it no longer used.

The current pattern of removing as aggressively as possible and then
taking another pass over the removed expressions to re-add seems to
be the most optimal approach.

Fixes: #2045
Signed-off-by: Patrick East <east.patrick@gmail.com>