[turbofan] Introduce CheckString simplified operator.

src/compiler/typer.cc

Index: src/compiler/typer.cc
diff --git a/src/compiler/typer.cc b/src/compiler/typer.cc
index 98cf98ac337a6a1325452742970fc56eba1d3906..c22a0d477c271015d29ce5f9d54cae5b26e8178e 100644
--- a/src/compiler/typer.cc
+++ b/src/compiler/typer.cc
@@ -1806,6 +1806,11 @@ Type* Typer::Visitor::TypeCheckNumber(Node* node) {
   return Type::Intersect(arg, Type::Number(), zone());
 }
 
+Type* Typer::Visitor::TypeCheckString(Node* node) {
+  Type* arg = Operand(node, 0);
+  return Type::Intersect(arg, Type::String(), zone());
+}
+
 Type* Typer::Visitor::TypeCheckIf(Node* node) {
   UNREACHABLE();
   return nullptr;