Function has_users 

pub fn has_users<'a, 'str: 'a>(
    host: impl QCodeView<'a, 'str>,
    v: ValueId,
) -> bool

Whether this host records any user of v. A shared value with no owning function has none. Mirrors Context::has_users.

Deliberately not users_of(v).is_empty(): this is asked once per instruction per round, and building a vector only to discard it was the largest single cost of lifting a block.