Bug 268870 – [Wasm-GC] Fix handling of bottom type in struct ops

• Component: WebAssembly
• Status: RESOLVED FIXED

[Asumu Takikawa]

Some validation functions don't handle the bottom type (ref null none) or other types quite right due to assuming that a struct index is present.

(values of the bottom type inhabit all of the concrete types)

A test case:

```
diff --git a/JSTests/wasm/gc/structs.js b/JSTests/wasm/gc/structs.js
index e1b24e9b1572..4a67c6748580 100644
--- a/JSTests/wasm/gc/structs.js
+++ b/JSTests/wasm/gc/structs.js
@@ -751,6 +751,19 @@ function testStructGet() {
     WebAssembly.RuntimeError,
     "struct.get to a null reference"
   );
+
+  // Bottom is type valid but throws due to null.
+  assert.throws(
+    () => instantiate(`
+      (module
+        (type (struct (field i32)))
+        (func (export "f") (result i32)
+          (struct.get 0 0 (ref.null none)))
+      )
+    `).exports.f(),
+    WebAssembly.RuntimeError,
+    "struct.get to a null reference"
+  );
 }
```

[Asumu Takikawa]

Pull request: https://github.com/WebKit/WebKit/pull/24019

[EWS]

Committed 274556@main (d27f9e79585d): <https://commits.webkit.org/274556@main>

Reviewed commits have been landed. Closing PR #24019 and removing active labels.

[Radar WebKit Bug Importer]

<rdar://problem/122890975>