change undefined == x to null == x

fixes #2871

lib/compress.js

@@ -4858,6 +4858,12 @@ merge(Compressor.prototype, {
             }
             break;
         }
+        // void 0 == x => null == x
+        if (compressor.option("comparisons") &&
+            (self.operator == "==" || self.operator == "!=") &&
+            is_undefined(self.left, compressor)) {
+            self.left = make_node_from_constant(null, self.left);
+        }
         if (self.operator == "+" && compressor.in_boolean_context()) {
             var ll = self.left.evaluate(compressor);
             var rr = self.right.evaluate(compressor);

test/compress/issue-2871.js

@@ -0,0 +1,17 @@
+comparison_with_undefined: {
+    options = {
+        comparisons: true,
+    }
+    input: {
+        a == undefined
+        a != undefined
+        a === undefined
+        a !== undefined
+    }
+    expect: {
+        null == a
+        null != a
+        void 0 === a
+        void 0 !== a
+    }
+}