simplify do-while into for (#2907)

fixes #2904

lib/compress.js

@@ -3672,6 +3672,13 @@ merge(Compressor.prototype, {
                 ]
             }).optimize(compressor);
         }
+        if (self.body instanceof AST_SimpleStatement) return make_node(AST_For, self, {
+            condition: make_sequence(self.condition, [
+                self.body.body,
+                self.condition
+            ]),
+            body: make_node(AST_EmptyStatement, self)
+        }).optimize(compressor);
         return self;
     });