Switching arguments to recv_if results in confusing error message

Interchanging the bool and channel in recv_if in DSLX results in an unhelpful message.

```
pub proc foo {
in_ch: chan in;
out_ch: chan out;

init { () }

config () {
let (i, o) = chan;
(i, o)
}

next(tok: token, state: ()) {
let (tok, _) = recv_if(tok, false, in_ch);
let tok = send(tok, out_ch, true);
()
}
}
```

```
$ bazel-bin/third_party/xls/dslx/interpreter_main input.x
...
E0113 14:22:29.462753 2783768 deduce.cc:2217] INTERNAL: XLS_RET_CHECK failure (third_party/xls/dslx/deduce.cc:2217) index != nullptr
```