[turbofan] Add new StringFromCharCode simplified operator.

src/compiler/typer.cc

Index: src/compiler/typer.cc
diff --git a/src/compiler/typer.cc b/src/compiler/typer.cc
index 29cbc57959e0ebd90ac0a926d9716809aa94459d..54ea26314e276a7bbd76feaf17253f801e5df45c 100644
--- a/src/compiler/typer.cc
+++ b/src/compiler/typer.cc
@@ -272,6 +272,7 @@ class Typer::Visitor : public Reducer {
   static Type* JSCallFunctionTyper(Type*, Typer*);
 
   static Type* ReferenceEqualTyper(Type*, Type*, Typer*);
+  static Type* StringFromCharCodeTyper(Type*, Typer*);
 
   Reduction UpdateType(Node* node, Type* current) {
     if (NodeProperties::IsTyped(node)) {
@@ -1814,6 +1815,23 @@ Type* Typer::Visitor::TypeStringLessThanOrEqual(Node* node) {
   return Type::Boolean();
 }
 
+Type* Typer::Visitor::StringFromCharCodeTyper(Type* type, Typer* t) {
+  type = NumberToUint32(ToNumber(type, t), t);
+  Factory* f = t->isolate()->factory();
+  double min = type->Min();
+  double max = type->Max();
+  if (min == max) {
+    uint32_t code = static_cast<uint32_t>(min) & String::kMaxUtf16CodeUnitU;
+    Handle<String> string = f->LookupSingleCharacterStringFromCode(code);
+    return Type::Constant(string, t->zone());
+  }
+  return Type::String();
+}
+
+Type* Typer::Visitor::TypeStringFromCharCode(Node* node) {
+  return TypeUnaryOp(node, StringFromCharCodeTyper);
+}
+
 Type* Typer::Visitor::TypeStringToNumber(Node* node) {
   return TypeUnaryOp(node, ToNumber);
 }