[turbofan] Introduce CheckIf simplified operator.

src/compiler/typer.cc

Index: src/compiler/typer.cc
diff --git a/src/compiler/typer.cc b/src/compiler/typer.cc
index cdd1fdd0aa4c3c85fe9bbc75141508df54da578a..382ee8a4aa43b69013ad66f03881efc9ed2113cc 100644
--- a/src/compiler/typer.cc
+++ b/src/compiler/typer.cc
@@ -1746,6 +1746,11 @@ Type* Typer::Visitor::TypeCheckNumber(Node* node) {
   return Type::Intersect(arg, Type::Number(), zone());
 }
 
+Type* Typer::Visitor::TypeCheckIf(Node* node) {
+  UNREACHABLE();
+  return nullptr;
+}
+
 Type* Typer::Visitor::TypeCheckTaggedPointer(Node* node) {
   Type* arg = Operand(node, 0);
   return Type::Intersect(arg, Type::TaggedPointer(), zone());