Avoid syntax error in yield assignments (fixes #1054)

lib/output.js

@@ -1234,9 +1234,7 @@ function OutputStream(options) {
         }
 
         isYield = (self.right.operator == "yield" || self.right.operator === "yield*");
-        isYield && output.print("(");
         output.print(op);
-        isYield && output.print(")");
 
         if ((op == "<" || op == "<<")
             && self.right instanceof AST_UnaryPrefix

test/compress/harmony.js

@@ -434,3 +434,13 @@ generators_yield: {
     }
     expect_exact: "function*fn(){yield remote()}"
 }
+
+generators_yield_assign: {
+    input: {
+      function* fn() {
+          var x = {};
+          x.prop = yield 5;
+      }
+    }
+    expect_exact: "function*fn(){var x={};x.prop=yield 5}"
+}