[turbofan] Add support for String.prototype.charCodeAt/charAt.

src/compiler/typer.cc

Index: src/compiler/typer.cc
diff --git a/src/compiler/typer.cc b/src/compiler/typer.cc
index 2c95d2745bf844e8a185d7d4ee168d10f1443bdd..c2544a30704d145bbe5dac245a0a5d3c36b8f2ce 100644
--- a/src/compiler/typer.cc
+++ b/src/compiler/typer.cc
@@ -1883,6 +1883,11 @@ Type* Typer::Visitor::StringFromCharCodeTyper(Type* type, Typer* t) {
   return Type::String();
 }
 
+Type* Typer::Visitor::TypeStringCharCodeAt(Node* node) {
+  // TODO(bmeurer): We could do better here based on inputs.
+  return Type::Range(0, kMaxUInt16, zone());
+}
+
 Type* Typer::Visitor::TypeStringFromCharCode(Node* node) {
   return TypeUnaryOp(node, StringFromCharCodeTyper);
 }