implement delayed resolution for reduce_vars (#1788)

Although it would be nice to enforce `AST_Node` cloning during transformation, that ship has sailed a long time ago.

We now get the assigned value when resolving `AST_SymbolRef` instead of `reset_opt_flags()`, which has the added advantage of improved compressor efficiency.

fixes #1787

test/compress/issue-1609.js

@@ -45,11 +45,10 @@ chained_evaluation_2: {
     }
     expect: {
         (function() {
-            var a = "long piece of string";
             (function() {
-                var c;
-                c = f(a);
-                c.bar = a;
+                var c, b = "long piece of string";
+                c = f(b);
+                c.bar = b;
             })();
         })();
     }