[Wasm-GC] Fix returned reference in br_on_null in LLInt/B3

[Asumu Takikawa]

The following test case using `br_on_null` crashes on some tiers:

```
//@ runWebAssemblySuite("--useWebAssemblyTypedFunctionReferences=true", "--useWebAssemblyGC=true")

import * as assert from "../assert.js";
import { compile, instantiate } from "./wast-wrapper.js";

instantiate(`
(module
  (type (struct (field i32)))
  (func (export "f")
    (block
      (struct.new 0 (i32.const 42))
      (br_on_null 0)
      (struct.get 0 0)
      drop
      ))
)
`).exports.f();
```

It works fine in BBQJIT, but seems to have issues in LLInt (possibly OMG). The problem is the returned reference on a non-taken branch case is not handled quite right.

[Asumu Takikawa]

Pull request: https://github.com/WebKit/WebKit/pull/21692

[EWS]

Committed 271964@main (4c063193fca3): <https://commits.webkit.org/271964@main>

Reviewed commits have been landed. Closing PR #21692 and removing active labels.

[Radar WebKit Bug Importer]

<rdar://problem/119590935>