[turbofan] Improve typed lowering for JSToBoolean.

test/mjsunit/boolean.js

Index: test/mjsunit/boolean.js
diff --git a/test/mjsunit/boolean.js b/test/mjsunit/boolean.js
index d955855d3aa6a610f1ded3a09a195372015305ec..9b9edd2e019a4b6cab9bc2672e10d7ea00d8ea5b 100644
--- a/test/mjsunit/boolean.js
+++ b/test/mjsunit/boolean.js
@@ -72,3 +72,10 @@ 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));