enhance merge_vars (#5194)

closes #5182

lib/compress.js

@@ -203,6 +203,7 @@ merge(Compressor.prototype, {
         if (this.option("expression")) {
             node.process_expression(true);
         }
+        var merge_vars = this.options.merge_vars;
         var passes = +this.options.passes || 1;
         var min_count = 1 / 0;
         var stopping = false;
@@ -211,6 +212,7 @@ merge(Compressor.prototype, {
             node.figure_out_scope(mangle);
             if (pass > 0 || this.option("reduce_vars"))
                 node.reset_opt_flags(this);
+            this.options.merge_vars = merge_vars && (stopping || pass == passes - 1);
             node = node.transform(this);
             if (passes > 1) {
                 var count = 0;
@@ -259,8 +261,8 @@ merge(Compressor.prototype, {
         descend(node, this);
         var opt = node.optimize(this);
         if (is_scope && opt === node && !this.has_directive("use asm") && !opt.pinned()) {
-            opt.merge_variables(this);
             opt.drop_unused(this);
+            if (opt.merge_variables(this)) opt.drop_unused(this);
             descend(opt, this);
         }
         if (opt === node) opt._squeezed = true;
@@ -5994,6 +5996,7 @@ merge(Compressor.prototype, {
         });
         tw.directives = Object.create(compressor.directives);
         self.walk(tw);
+        var changed = false;
         var merged = Object.create(null);
         while (first.length && last.length) {
             var head = first.pop();
@@ -6037,10 +6040,12 @@ merge(Compressor.prototype, {
                 def.references = refs.concat(def.references);
                 def.fixed = tail.definition.fixed && def.fixed;
                 merged[id] = def;
+                changed = true;
                 break;
             } while (last.length);
             if (skipped.length) last = last.concat(skipped);
         }
+        return changed;
 
         function push() {
             segment = Object.create(segment);