[Wasm-GC] Fix recursive return type index for struct.new_default

[Asumu Takikawa]

Minimal test case (goes in `structs.js` test):

```
  compile(`
     (module
       (rec (type (struct (field (ref null 0)))))
       (func (param (ref null 0)))
       (func
         (struct.new_default 0 (ref.null 0))
         (call 0))
     )
  `);
```

errors with `wasm.yaml/wasm/gc/structs.js.wasm-no-cjit: ASSERTION FAILED: subRTT.has_value() && parentRTT.has_value()`

The cause is there's a type expansion done at the wrong time in validation for `struct.new_default`, and has a very simple fix.

[Asumu Takikawa]

Pull request: https://github.com/WebKit/WebKit/pull/22615

[EWS]

Committed 272941@main (1619546caa40): <https://commits.webkit.org/272941@main>

Reviewed commits have been landed. Closing PR #22615 and removing active labels.

[Radar WebKit Bug Importer]

<rdar://problem/120867628>