enhance varify (#5658)

lib/compress.js

@@ -10120,9 +10120,11 @@ Compressor.prototype.compress = function(node) {
     }
 
     function can_varify(compressor, sym) {
-        if (!sym.fixed_value()) return false;
         var def = sym.definition();
-        return is_safe_lexical(def) && same_scope(def) && !may_overlap(compressor, def);
+        return (def.fixed || def.fixed === 0)
+            && is_safe_lexical(def)
+            && same_scope(def)
+            && !may_overlap(compressor, def);
     }
 
     function varify(self, compressor) {

test/compress/varify.js

@@ -194,6 +194,43 @@ scope_adjustment_let: {
     node_version: ">=4"
 }
 
+escaped_const: {
+    options = {
+        reduce_vars: true,
+        toplevel: true,
+        varify: true,
+    }
+    input: {
+        const log = console.log;
+        log("PASS");
+    }
+    expect: {
+        var log = console.log;
+        log("PASS");
+    }
+    expect_stdout: "PASS"
+}
+
+escaped_let: {
+    options = {
+        reduce_vars: true,
+        toplevel: true,
+        varify: true,
+    }
+    input: {
+        "use strict";
+        let log = console.log;
+        log("PASS");
+    }
+    expect: {
+        "use strict";
+        var log = console.log;
+        log("PASS");
+    }
+    expect_stdout: "PASS"
+    node_version: ">=4"
+}
+
 issue_4191_const: {
     options = {
         functions: true,