[turbofan] Improve typed lowering for JSToBoolean.

test/mjsunit/boolean.js

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 54 matching lines @@
 assertEquals(void 0, t.p && (t.p == t.p));
 
 var o = new Object();
 o.p = 'foo';
 assertEquals('foo', o.p);
 assertEquals('foo', o.p || true);
 assertEquals('foo', o.p || false);
 assertEquals('foo', o.p || (o.p == 0));
 assertEquals('foo', o.p || (o.p == null));
 assertEquals('foo', o.p || (o.p == o.p));
+
+// JSToBoolean(x:string)
+function f(x) { return !!("" + x); }
+assertEquals(false, f(""));
+assertEquals(true, f("narf"));
+assertEquals(true, f(12345678));
+assertEquals(true, f(undefined));