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-1787.js

@@ -0,0 +1,19 @@
+unary_prefix: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        console.log(function() {
+            var x = -(2 / 3);
+            return x;
+        }());
+    }
+    expect: {
+        console.log(function() {
+            return -2 / 3;
+        }());
+    }
+    expect_stdout: true
+}