Preliminary support for labeled break/continue for loops

[catamorphism]

This patch adds preliminary middle-end support (liveness and trans)
for breaks and loops to loop constructs that have labels.

while and for loops can't have labels yet.

Progress on #2216

r? @pcwalton r+ from @nikomatsakis so no need for other review.

[nikomatsakis]

@catamorphism @pcwalton hope you don't mind, I was scanning the pull request queue and saw that this touched on liveness, so I wanted to take a look. Overall it looks pretty reasonable (modulo some nits regarding indentation).

I did have one question: Can you add a test that breaks out of a for loop, like this?

fn main() {
    let v = ~[1, 2, 3];
    loop foo: {
        for v.each |e| { break foo; }
        io::println("Hello, world!\n");
        break;
    }
    io::println("Goodbye, world!\n");
}

Based on my testing, this test will not execute correctly (that is, "Hello, World!" and "Goodye, World!" will both print). I think it's perfectly fine if we do not support using named breaks out of for loops (at least for the moment), but we should either report an error or Do The Right Thing. Of course I could be misreading the code, but I believe that resolve accepts this (at least in my local testing) and I also believe that if it gets all the way to trans, it will simply ignore the label and break out of the for loop. Actually though what I think will happen is that it will ICE in liveness, because the hashmap mapping ids to nodes will not contain anything for foo inside the closure.

[catamorphism]

I merged this.

[catamorphism]

(Oh, and in response to @nikomatsakis 's last comment, I'm working on this -- it's almost done.)