[wasm-gc][fuzzer] Fix call_ref with immediate

Use the correct helper function to emit an immediate.

Fixed: chromium:1358909
Change-Id: I2f2ae7819f40009b3f9c22067cdf11885d3347ca
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3869265Reviewed-by: Matthias Liedtke <mliedtke@chromium.org>
Auto-Submit: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Matthias Liedtke <mliedtke@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82924}

test/fuzzer/wasm-compile.cc

@@ -651,8 +651,7 @@ class WasmGenerator {
         builder_->EmitByte(0);  // Table index.
       } else {
         GenerateRef(HeapType(sig_index), data);
-        builder_->Emit(kExprReturnCallRef);
-        builder_->EmitI32Const(sig_index);
+        builder_->EmitWithU32V(kExprReturnCallRef, sig_index);
       }
       return;
     } else {
@@ -667,8 +666,7 @@ class WasmGenerator {
         builder_->EmitByte(0);  // Table index.
       } else {
         GenerateRef(HeapType(sig_index), data);
-        builder_->Emit(kExprCallRef);
-        builder_->EmitI32Const(sig_index);
+        builder_->EmitWithU32V(kExprCallRef, sig_index);
       }
     }
     if (sig->return_count() == 0 && wanted_kind != kWasmVoid) {