alias pass crashes when it needs to autoderef through multiple boxes

[msullivan]

type quux = rec(int bar);

fn g(&int i) { }
fn f(@@quux foo) {
    g(foo.bar);
}

fn main() {}

Yields

rt: 8994:main:main:                   upcall fail 'non-exhaustive match failure', ../src/comp/middle/alias.rs:546

It is hitting a non-exhaustive match because only one of the boxes around foo is being pulled away.