Malformed inline assembly rejected by LLVM.

LLVM checks the validity of the constraints and the assembly string passed to
it. This error implies that LLVM seems something wrong with the inline
assembly call.

In particular, it can happen if you forgot the closing bracket of a register
constraint (see issue #51430):
```ignore (error-emitted-at-codegen-which-cannot-be-handled-by-compile_fail)
#![feature(asm)]

fn main() {
    let rax: u64;
    unsafe {
        asm!("" :"={rax"(rax));
        println!("Accumulator is: {}", rax);
    }
}
```
