An existing test, https://searchfox.org/wubkat/source/JSTests/wasm/gc/casts.js , fails in the wasm-simd, wasm-bbq, and wasm-air configurations when running the run-jsc-stress-tests script. I'm not sure why the test isn't being run with these configurations in CI.

The output is:
```
wasm.yaml/wasm/gc/casts.js.wasm-simd: ASSERTION FAILED: isSubtype(src.type(), dst.type())
wasm.yaml/wasm/gc/casts.js.wasm-simd: /home/tjc/WebKit/Source/JavaScriptCore/wasm/WasmAirIRGenerator64.cpp(701) : void JSC::Wasm::AirIRGenerator64::emitMove(const JSC::Wasm::TypedTmp&, const JSC::Wasm::TypedTmp&)
```

and the specific test case causing the error is the first one in `testNullCasts()`:

```
        (func (param arrayref) (result nullref)
          (ref.cast none (local.get 0)))
```

I'll submit a PR.