Never mangle arguments and keep them in their scope

Fixes #892

Helped-by: kzc

test/compress/issue-892.js

@@ -0,0 +1,32 @@
+dont_mangle_arguments: {
+    mangle = {
+    };
+    options = {
+        sequences     : true,
+        properties    : true,
+        dead_code     : true,
+        drop_debugger : true,
+        conditionals  : true,
+        comparisons   : true,
+        evaluate      : true,
+        booleans      : true,
+        loops         : true,
+        unused        : true,
+        hoist_funs    : true,
+        keep_fargs    : true,
+        keep_fnames   : false,
+        hoist_vars    : true,
+        if_return     : true,
+        join_vars     : true,
+        cascade       : true,
+        side_effects  : true,
+        negate_iife   : false
+    };
+    input: {
+        (function(){
+            var arguments = arguments, not_arguments = 9;
+            console.log(not_arguments, arguments);
+        })(5,6,7);
+    }
+    expect_exact: "(function(){var arguments=arguments,o=9;console.log(o,arguments)})(5,6,7);"
+}

test/run-tests.js

@@ -103,6 +103,10 @@ function run_compress_tests() {
             }
             var output = input.transform(cmp);
             output.figure_out_scope();
+            if (test.mangle) {
+                output.compute_char_frequency(test.mangle);
+                output.mangle_names(test.mangle);
+            }
             output = make_code(output, output_options);
             if (expect != output) {
                 log("!!! failed\n---INPUT---\n{input}\n---OUTPUT---\n{output}\n---EXPECTED---\n{expected}\n\n", {