[turbofan] Improve typed lowering for JSToBoolean.

test/cctest/compiler/test-js-typed-lowering.cc

Index: test/cctest/compiler/test-js-typed-lowering.cc
diff --git a/test/cctest/compiler/test-js-typed-lowering.cc b/test/cctest/compiler/test-js-typed-lowering.cc
index d48a462c1f3b38a95d4b353eaed548cfa5f10636..8a78c9386e27dfa03dc8bdb2acf4c86b89b897b1 100644
--- a/test/cctest/compiler/test-js-typed-lowering.cc
+++ b/test/cctest/compiler/test-js-typed-lowering.cc
@@ -508,8 +508,12 @@ TEST(JSToBoolean) {
 
   {  // ToBoolean(string)
     Node* r = R.ReduceUnop(op, Type::String());
-    // TODO(titzer): test will break with better js-typed-lowering
-    CHECK_EQ(IrOpcode::kJSToBoolean, r->opcode());
+    CHECK_EQ(IrOpcode::kBooleanNot, r->opcode());
+    Node* i = r->InputAt(0);
+    CHECK_EQ(IrOpcode::kNumberEqual, i->opcode());
+    Node* j = i->InputAt(0);
+    CHECK_EQ(IrOpcode::kLoadField, j->opcode());
+    // ToBoolean(x:string) => BooleanNot(NumberEqual(x.length, #0))
   }
 
   {  // ToBoolean(object)