fix corner case with collapse_vars & ie8 (#3469)

fixes #3468

lib/compress.js

@@ -896,8 +896,7 @@ merge(Compressor.prototype, {
         if (lhs instanceof AST_This) return true;
         if (lhs instanceof AST_SymbolRef) {
             var def = lhs.definition();
-            return def.orig[0] instanceof AST_SymbolLambda
-                || compressor.exposed(def) && identifier_atom[def.name];
+            return def.lambda || compressor.exposed(def) && identifier_atom[def.name];
         }
         if (lhs instanceof AST_PropAccess) {
             lhs = lhs.expression;

lib/scope.js

@@ -55,6 +55,7 @@ function SymbolDef(scope, orig, init) {
     this.mangled_name = null;
     this.undeclared = false;
     this.id = SymbolDef.next_id++;
+    this.lambda = orig instanceof AST_SymbolLambda;
 }
 
 SymbolDef.next_id = 1;
@@ -210,6 +211,7 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options) {
             node.thedef = new_def;
             node.reference(options);
         });
+        if (old_def.lambda) new_def.lambda = true;
     }
 });