Simplify nested conditionals if possible

lib/compress.js

@@ -2296,6 +2296,19 @@ merge(Compressor.prototype, {
                 return consequent;
             }
         }
+        // x?y?z:a:a --> x&&y?z:a
+        if (consequent instanceof AST_Conditional
+            && consequent.alternative.equivalent_to(alternative)) {
+            return make_node(AST_Conditional, self, {
+                condition: make_node(AST_Binary, self, {
+                    left: self.condition,
+                    operator: "&&",
+                    right: consequent.condition
+                }),
+                consequent: consequent.consequent,
+                alternative: alternative
+            });
+        }
         return self;
     });