[turbofan] Introduce CheckPoint common operator.

src/compiler/typer.cc

Index: src/compiler/typer.cc
diff --git a/src/compiler/typer.cc b/src/compiler/typer.cc
index 75628517331e9d6562ae8e4377404b3db88550a7..1a7e99d78217139ab5ad09aba1b1048c76d3f7cd 100644
--- a/src/compiler/typer.cc
+++ b/src/compiler/typer.cc
@@ -706,6 +706,10 @@ Type* Typer::Visitor::TypeGuard(Node* node) {
   return Type::Intersect(input_type, guard_type, zone());
 }
 
+Type* Typer::Visitor::TypeCheckPoint(Node* node) {
+  UNREACHABLE();
+  return nullptr;
+}
 
 Type* Typer::Visitor::TypeBeginRegion(Node* node) {
   UNREACHABLE();