fix corner case in collapse_vars (#4243)

fixes #4242

lib/compress.js

@@ -1974,6 +1974,7 @@ merge(Compressor.prototype, {
             }
 
             function foldable(expr) {
+                if (expr instanceof AST_Assign && expr.right.single_use) return;
                 var lhs_ids = Object.create(null);
                 var marker = new TreeWalker(function(node) {
                     if (node instanceof AST_SymbolRef) lhs_ids[node.definition().id] = true;