Remove duplicated code from comma-separated Expression parsing

src/parsing/parser.h

Index: src/parsing/parser.h
diff --git a/src/parsing/parser.h b/src/parsing/parser.h
index 18f9b78d657b94cc89242ece08f6923420a7c6d1..4485c3a673f7af553fc1d554a477882f95aa2389 100644
--- a/src/parsing/parser.h
+++ b/src/parsing/parser.h
@@ -879,6 +879,10 @@ class Parser : public ParserBase<Parser> {
   }
   V8_INLINE static FunctionLiteral* EmptyFunctionLiteral() { return nullptr; }
 
+  V8_INLINE static bool IsEmptyExpression(Expression* expr) {
+    return expr == nullptr;
+  }
+
   // Used in error return values.
   V8_INLINE static ZoneList<Expression*>* NullExpressionList() {
     return nullptr;