enhance conditionals (#2966)

- `x ? (y, w) : (z, w)` => `x ? y : z, w`

lib/compress.js

@@ -5727,6 +5727,18 @@ merge(Compressor.prototype, {
                 consequent
             ]).optimize(compressor);
         }
+        // x ? (y, w) : (z, w) --> x ? y : z, w
+        if ((consequent instanceof AST_Sequence || alternative instanceof AST_Sequence)
+            && consequent.tail_node().equivalent_to(alternative.tail_node())) {
+            return make_sequence(self, [
+                make_node(AST_Conditional, self, {
+                    condition: self.condition,
+                    consequent: pop_seq(consequent),
+                    alternative: pop_seq(alternative)
+                }),
+                consequent.tail_node()
+            ]).optimize(compressor);
+        }
         // x ? y || z : z --> x && y || z
         if (consequent instanceof AST_Binary
             && consequent.operator == "||"
@@ -5829,6 +5841,13 @@ merge(Compressor.prototype, {
                 }
             }
         }
+
+        function pop_seq(node) {
+            if (!(node instanceof AST_Sequence)) return make_node(AST_Number, node, {
+                value: 0
+            });
+            return make_sequence(node, node.expressions.slice(0, -1));
+        }
     });
 
     OPT(AST_Boolean, function(self, compressor){