Cranelift: ISLE extraction panics

When I create a new extractor, one to extract the comparison flag from a comparison referenced by `select`, ISLE panics.

### Steps to Reproduce

Add:

```
(decl select_fcmp_flag (FloatCC) Inst)
(extractor (select_fcmp_flag comparison_flag)
(select (def_inst (fcmp comparison_flag a b)) x y))
```

Then run:

```
cargo build -p cranelift-codegen --features rebuild-isle
```

### Expected Results

The ISLE code to generate without failure.

### Actual Results

```
--- stderr
thread 'main' panicked at 'assertion failed: !tyenv.errors.is_empty()', cranelift/isle/isle/src/sema.rs:1055:29
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

### Versions and Environment

Cranelift version or commit: latest `HEAD`