[WASM-Function-References] Fix block signature parsing for reftypes

[Asumu Takikawa]

Parsing of block signatures currently does not correctly account for ref types with type indices. Here is an example that illustrates the problem:


```
// Run this in JSTests/wasm/gc

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

  instantiate(`
(module
  (type (func))

  (func (export "run")
    (block (result (ref null func)) (ref.null 0)) ;; (ref null 0) <: (ref null func)
    (br 0)
  )
)
`).exports.run();
```

This program should validate (and does in the reference interpreter) but currently doesn't in JSC. The problem is that `(ref null func)` in the block signature is not parsed correctly; it gets turned into a ref type with an invalid 0 index.

This can be fixed by adding a special case to `parseBlockSignature` for ref types.

[Radar WebKit Bug Importer]

<rdar://problem/102159543>

[Asumu Takikawa]

Pull request: https://github.com/WebKit/WebKit/pull/9081

[EWS]

Committed 259421@main (3e28bdb07aa2): <https://commits.webkit.org/259421@main>

Reviewed commits have been landed. Closing PR #9081 and removing active labels.

[WebKit Commit Bot]

Re-opened since this is blocked by bug 251295

[Asumu Takikawa]

Pull request: https://github.com/WebKit/WebKit/pull/16542

[EWS]

Committed 266847@main (5ff67f8945d7): <https://commits.webkit.org/266847@main>

Reviewed commits have been landed. Closing PR #16542 and removing active labels.

[WebKit Commit Bot]

Re-opened since this is blocked by bug 260123

[Asumu Takikawa]

Pull request: https://github.com/WebKit/WebKit/pull/17146

[EWS]

Committed 271262@main (9a518a553026): <https://commits.webkit.org/271262@main>

Reviewed commits have been landed. Closing PR #17146 and removing active labels.