[turbofan] Improve typed lowering rules for JSToBoolean.

test/unittests/compiler/graph-unittest.cc

Index: test/unittests/compiler/graph-unittest.cc
diff --git a/test/unittests/compiler/graph-unittest.cc b/test/unittests/compiler/graph-unittest.cc
index dc2ba7814ba6bd96538cde619c157cf536facfaa..37f3688891b1a9681f5e7f88b75e2d2e9b91cd83 100644
--- a/test/unittests/compiler/graph-unittest.cc
+++ b/test/unittests/compiler/graph-unittest.cc
@@ -98,6 +98,9 @@ Matcher<Node*> GraphTest::IsTrueConstant() {
   return IsHeapConstant(factory()->true_value());
 }
 
+Matcher<Node*> GraphTest::IsNullConstant() {
+  return IsHeapConstant(factory()->null_value());
+}
 
 Matcher<Node*> GraphTest::IsUndefinedConstant() {
   return IsHeapConstant(factory()->undefined_value());