fix corner case in hoist_props & unused (#5455)

fixes #5454

lib/compress.js

@@ -7240,6 +7240,7 @@ Compressor.prototype.compress = function(node) {
                             });
                             var index = indexOf_assign(sym, def);
                             if (index >= 0) assign_in_use[sym.id][index] = assign;
+                            sym.assignments++;
                             sym.eliminated++;
                             return assign;
                         }));