FFI: If local dslx implementation of a FFI function contains assert, optimizer complains as tokens are not passed

To test, let's modify the ffi example by adding an assert (`fail!()`):

```diff
--- a/xls/examples/ffi.x
+++ b/xls/examples/ffi.x
@@ -39,7 +39,7 @@
fn divmod(a:bits[A_WIDTH], b:bits[B_WIDTH])
-> (bits[A_WIDTH], bits[B_WIDTH], bool) {
if (b == u32:0) {
- (a, b, true)
+ fail!("division_by_zero", (a, b, true))
} else {
(a / b, a % b, false)
}
```

Then if we trigger an ir conversion and ir optimization:
```
bazel build xls/examples:ffi_codegen
```

... the `opt_main` will fail with
```
Error: INTERNAL: Node type contains token type even though it shouldn't: invoke.56: (token, (bits[32], bits[32], bits[1])) = invoke(after_all.34, literal.35, dividend, divisor, to_apply=__itok__ffi__divmod__32_32, id=56, pos=[(0,49,8)])
```

This is probably because the FFI function is not inlined and this creates some missed assumptions later on.