[fuzzer] Use recursion scope only for nullable references in GenerateRef

Use recursion scope only for nullable references in the
GenerateRef function. We declare the recursion scope as
an optional and only initialize it if the reference type
is nullable.

Bug: v8:11954, chromium:1270126
Change-Id: I1548290cc9d48167f6fd56ff653744d472f65635
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3284894Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Manos Koukoutos <manoskouk@chromium.org>
Commit-Queue: Maria Tîmbur <mtimbur@google.com>
Cr-Commit-Position: refs/heads/main@{#77943}

test/fuzzer/wasm-compile.cc

@@ -2065,7 +2065,11 @@ void WasmGenerator::Generate(ValueType type, DataRange* data) {
 
 void WasmGenerator::GenerateRef(HeapType type, DataRange* data,
                                 Nullability nullability) {
-  GeneratorRecursionScope rec_scope(this);
+  base::Optional<GeneratorRecursionScope> rec_scope;
+  if (nullability) {
+    rec_scope.emplace(this);
+  }
+
   if (recursion_limit_reached() || data->size() == 0) {
     if (nullability == kNullable) {
       ref_null(type, data);