Remove AssignmentExpressionFlags enum, handle error checking in callers

src/parsing/preparser.h

Index: src/parsing/preparser.h
diff --git a/src/parsing/preparser.h b/src/parsing/preparser.h
index 32b6446399d7f4af3cbc49441137ca11cbb44b1a..fc187c4c101f4b094fd95a42829a03e10825ba41 100644
--- a/src/parsing/preparser.h
+++ b/src/parsing/preparser.h
@@ -197,6 +197,8 @@ class PreParserExpression {
            ExpressionTypeField::decode(code_) == kAssignment;
   }
 
+  bool IsRewritableAssignmentExpression() const { return IsAssignment(); }
+
   bool IsObjectLiteral() const {
     return TypeField::decode(code_) == kObjectLiteralExpression;
   }