[turbofan] Improve JSToBoolean typing.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/792013006

Cr-Commit-Position: refs/heads/master@{#25896}

src/compiler/typer.cc

@@ -1182,7 +1182,7 @@ Bounds Typer::Visitor::TypeJSTypeOf(Node* node) {
 
 
 Bounds Typer::Visitor::TypeJSToBoolean(Node* node) {
-  return Bounds(Type::None(zone()), Type::Boolean(zone()));
+  return TypeUnaryOp(node, ToBoolean);
 }