LLVM assertion with asm! output constraint for "d" on x86 and x86_64

[emberian]

#![feature(asm)]

fn main() {
    let a: usize;

    unsafe {
        asm!("" : "=d"(a) : : : );
    }
}

vagrant@vagrant-ubuntu-trusty-64:~$ rustc reg-repro.rs 
reg-repro.rs:4:9: 4:10 warning: variable `a` is assigned to, but never used, #[warn(unused_variables)] on by default
reg-repro.rs:4     let a: usize;
                       ^
reg-repro.rs:7:24: 7:25 warning: value assigned to `a` is never read, #[warn(unused_assignments)] on by default
reg-repro.rs:7         asm!("" : "=d"(a) : : : );
                                      ^
note: in expansion of asm!
reg-repro.rs:7:9: 7:35 note: expansion site
rustc: /home/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/llvm/include/llvm/CodeGen/FunctionLoweringInfo.h:169: unsigned int llvm::FunctionLoweringInfo::InitializeRegForValue(const llvm::Value*): Assertion `R == 0 && "Already initialized this value register!"' failed.

[nagisa]

Relevant discussion

[jooert]

Triage: This doesn't assert anymore but gives a proper error while compiling (see https://is.gd/zIn5HT).

[Mark-Simulacrum]

Reproduced today, and as above, no LLVM assertion. E-needstest.