Bug 247383

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.