LLVM assert with plain "put;"

In generic-iter-frame.rs, replacing "put ();" with "put;" results in an LLVM assertion.

Reduced testcase:

```
iter i() { put; }
fn main() { for each j in i() { } }
```

Assertion failed: (((Args.size() == FTy->getNumParams()) || (FTy->isVarArg() && Args.size() > FTy->getNumParams())) && "Invoking a function with bad signature"), function init, file Instructions.cpp, line 526.
