Unreachable generic array concat breaks trans

Testcase 1:

```
fn f<T: copy>(a: T) {
    fail + [a]
}
fn main() {}
```

Testcase 2:

```
fn f<T>(g: fn() -> [T]) {
    let r = fail;
    r += g();
}
fn main() {}
```

Both trigger:

```
Assertion failed: (isa<X>(Val) && "cast<Ty>() argument of incompatible type!"), function cast, file /Users/jruderman/code/rust/src/llvm/include/llvm/Support/Casting.h, line 194.
```
