removing eager infer var replacement improves type inference

[lcnr]

the following test compiles with the new solver, but not with the old one. This now works because we don't do eager infer var replacement and use deferred projection equality.

trait Id {
    type Assoc;
}

impl<T> Id for T {
    type Assoc = T;
}

trait Trait<T> {}
impl Trait<u32> for Vec<u32> {}
impl Trait<i32> for Vec<u32> {}

fn assoc_pair<T: Id>() -> Option<*mut (Vec<T>, T::Assoc)> {
    None
}

fn impls_trait<T: Trait<U>, U>(_: Option<*mut (T, U)>) {}

fn main() {
    impls_trait(assoc_pair());
}