fix AST_Binary.lift_sequences() (#1621)

Commit eab99a1c fails to account for side effects from compound assignments.

lib/compress.js

@@ -2999,14 +2999,6 @@ merge(Compressor.prototype, {
         return self;
     });
 
-    function has_side_effects_or_prop_access(node, compressor) {
-        var save_pure_getters = compressor.option("pure_getters");
-        compressor.options.pure_getters = false;
-        var ret = node.has_side_effects(compressor);
-        compressor.options.pure_getters = save_pure_getters;
-        return ret;
-    }
-
     AST_Binary.DEFMETHOD("lift_sequences", function(compressor){
         if (compressor.option("sequences")) {
             if (this.left instanceof AST_Seq) {
@@ -3014,18 +3006,23 @@ merge(Compressor.prototype, {
                 var x = seq.to_array();
                 this.left = x.pop();
                 x.push(this);
-                seq = AST_Seq.from_array(x).transform(compressor);
-                return seq;
+                return AST_Seq.from_array(x).optimize(compressor);
             }
-            if (this.right instanceof AST_Seq
-                && this instanceof AST_Assign
-                && !has_side_effects_or_prop_access(this.left, compressor)) {
-                var seq = this.right;
-                var x = seq.to_array();
-                this.right = x.pop();
-                x.push(this);
-                seq = AST_Seq.from_array(x).transform(compressor);
-                return seq;
+            if (this.right instanceof AST_Seq && !this.left.has_side_effects(compressor)) {
+                var assign = this.operator == "=" && this.left instanceof AST_SymbolRef;
+                var root = this.right;
+                var cursor, seq = root;
+                while (assign || !seq.car.has_side_effects(compressor)) {
+                    cursor = seq;
+                    if (seq.cdr instanceof AST_Seq) {
+                        seq = seq.cdr;
+                    } else break;
+                }
+                if (cursor) {
+                    this.right = cursor.cdr;
+                    cursor.cdr = this;
+                    return root.optimize(compressor);
+                }
             }
         }
         return this;

test/compress/sequences.js

@@ -103,15 +103,18 @@ lift_sequences_1: {
 lift_sequences_2: {
     options = { sequences: true, evaluate: true };
     input: {
-        var foo, bar;
+        var foo = 1, bar;
         foo.x = (foo = {}, 10);
         bar = (bar = {}, 10);
+        console.log(foo, bar);
     }
     expect: {
-        var foo, bar;
+        var foo = 1, bar;
         foo.x = (foo = {}, 10),
-        bar = {}, bar = 10;
+        bar = {}, bar = 10,
+        console.log(foo, bar);
     }
+    expect_stdout: true
 }
 
 lift_sequences_3: {
@@ -138,6 +141,23 @@ lift_sequences_4: {
     }
 }
 
+lift_sequences_5: {
+    options = {
+        sequences: true,
+    }
+    input: {
+        var a = 2, b;
+        a *= (b, a = 4, 3);
+        console.log(a);
+    }
+    expect: {
+        var a = 2, b;
+        b, a *= (a = 4, 3),
+        console.log(a);
+    }
+    expect_stdout: "6"
+}
+
 for_sequences: {
     options = { sequences: true };
     input: {