Prepare for GC a little by generalizing heap type writing code

Author: kripken

src/wasm/wasm-binary.cpp

@@ -955,15 +955,13 @@ void WasmBinaryWriter::finishUp() {
 void WasmBinaryWriter::writeType(Type type) {
   if (type.isRef()) {
     auto heapType = type.getHeapType();
-    if (heapType.isSignature()) {
-      if (type.isNullable()) {
-        o << S32LEB(BinaryConsts::EncodedType::nullable);
-      } else {
-        o << S32LEB(BinaryConsts::EncodedType::nonnullable);
-      }
-      writeHeapType(heapType);
-      return;
+    if (type.isNullable()) {
+      o << S32LEB(BinaryConsts::EncodedType::nullable);
+    } else {
+      o << S32LEB(BinaryConsts::EncodedType::nonnullable);
     }
+    writeHeapType(heapType);
+    return;
   }
   int ret = 0;
   TODO_SINGLE_COMPOUND(type);