I.e. we should not prematurely build operand constraints at the expansion time. Otherwise `--pretty expanded` diverges:
```
$ cat t.rs
#![feature(asm)]
pub fn main() { unsafe { asm!("" : : : "hello", "world") }; }
$ rustc t.rs --pretty
#![feature(asm)]
pub fn main() { unsafe { asm!("" : : : "hello" , "world") }; }
$ rustc t.rs --pretty expanded
#![feature(asm)]
#![feature(phase)]
#![no_std]
#![feature(globs)]
#[phase(plugin, link)]
extern crate "std" as std;
#[prelude_import]
use std::prelude::*;
pub fn main() { unsafe { asm!("": : : "~{hello},~{world}") }; }
```
(The last code *does* compile, but won't do the expected thing.)