enhance passes (#2229)

- remove hardcoded upper limit
- continue based on node count reduction
- emit verbose statistics

fixes #2226

lib/compress.js

@@ -148,10 +148,20 @@ merge(Compressor.prototype, {
             node.process_expression(true);
         }
         var passes = +this.options.passes || 1;
-        for (var pass = 0; pass < passes && pass < 3; ++pass) {
+        var last_count = 1 / 0;
+        for (var pass = 0; pass < passes; pass++) {
             if (pass > 0 || this.option("reduce_vars"))
                 node.reset_opt_flags(this, true);
             node = node.transform(this);
+            if (passes > 1) {
+                var count = 0;
+                node.walk(new TreeWalker(function() {
+                    count++;
+                }));
+                this.info("pass " + pass + ": last_count: " + last_count + ", count: " + count);
+                if (count >= last_count) break;
+                last_count = count;
+            }
         }
         if (this.option("expression")) {
             node.process_expression(false);