trans trips assertion when using result of bottom-typed do-while

[msullivan]

fn main() {
    let x: int = do { fail } while (true);
}

results in

rt: 439d:main:main:                   upcall fail 'Assertion !*terminated failed', ../src/comp/lib/llvm.rs:927

Tim, you've fixed this for a bunch of other cases; do you have time to take this one?

[catamorphism]

Sure, I'll look at it.