drop unused function arguments

also add test for "drop_unused" (the last one fails for now)

test/compress/drop-unused.js

@@ -0,0 +1,97 @@
+unused_funarg_1: {
+    options = { unused: true };
+    input: {
+        function f(a, b, c, d, e) {
+            return a + b;
+        }
+    }
+    expect: {
+        function f(a, b) {
+            return a + b;
+        }
+    }
+}
+
+unused_funarg_2: {
+    options = { unused: true };
+    input: {
+        function f(a, b, c, d, e) {
+            return a + c;
+        }
+    }
+    expect: {
+        function f(a, b, c) {
+            return a + c;
+        }
+    }
+}
+
+unused_nested_function: {
+    options = { unused: true };
+    input: {
+        function f(x, y) {
+            function g() {
+                something();
+            }
+            return x + y;
+        }
+    };
+    expect: {
+        function f(x, y) {
+            return x + y;
+        }
+    }
+}
+
+unused_circular_references_1: {
+    options = { unused: true };
+    input: {
+        function f(x, y) {
+            // circular reference
+            function g() {
+                return h();
+            }
+            function h() {
+                return g();
+            }
+            return x + y;
+        }
+    };
+    expect: {
+        function f(x, y) {
+            return x + y;
+        }
+    }
+}
+
+unused_circular_references_2: {
+    options = { unused: true };
+    input: {
+        function f(x, y) {
+            var foo = 1, bar = baz, baz = foo + bar, qwe = moo();
+            return x + y;
+        }
+    };
+    expect: {
+        function f(x, y) {
+            moo();              // keeps side effect
+            return x + y;
+        }
+    }
+}
+
+unused_circular_references_3: {
+    options = { unused: true };
+    input: {
+        function f(x, y) {
+            var g = function() { return h() };
+            var h = function() { return g() };
+            return x + y;
+        }
+    };
+    expect: {
+        function f(x, y) {
+            return x + y;
+        }
+    }
+}