More attempts to determine when addition is associative

Somebody hit me with bug reports on this. :)

Refs #300

test/compress/arrays.js

@@ -69,6 +69,6 @@ constant_join_2: {
         var e = [ "foo", "bar", boo(),
                   "foo+1+2+3+bar",
                   "baz", "x", "y" ].join("really-long-separator");
-        var f = "strstr" + variable + "foobar" + ("moo" + foo);
+        var f = "strstr" + variable + "foobarmoo" + foo;
     }
 }

test/compress/concat-strings.js

@@ -0,0 +1,22 @@
+concat_1: {
+    options = {
+        evaluate: true
+    };
+    input: {
+        var a = "foo" + "bar" + x() + "moo" + "foo" + y() + "x" + "y" + "z" + q();
+        var b = "foo" + 1 + x() + 2 + "boo";
+        var c = 1 + x() + 2 + "boo";
+
+        // this CAN'T safely be shortened to 1 + x() + "5boo"
+        var d = 1 + x() + 2 + 3 + "boo";
+
+        var e = 1 + x() + 2 + "X" + 3 + "boo";
+    }
+    expect: {
+        var a = "foobar" + x() + "moofoo" + y() + "xyz" + q();
+        var b = "foo1" + x() + "2boo";
+        var c = 1 + x() + 2 + "boo";
+        var d = 1 + x() + 2 + 3 + "boo";
+        var e = 1 + x() + 2 + "X3boo";
+    }
+}