allow symbol replacement on multiple occurrences (#2472)

- all-or-nothing replacement
- avoid unmangleable names

fixes #2436

lib/scope.js

@@ -46,8 +46,10 @@
 function SymbolDef(scope, index, orig) {
     this.name = orig.name;
     this.orig = [ orig ];
+    this.eliminated = 0;
     this.scope = scope;
     this.references = [];
+    this.replaced = 0;
     this.global = false;
     this.mangled_name = null;
     this.undeclared = false;