[turbofan] Introduce a dedicated StringCharAt operator.

src/compiler/typer.cc

Index: src/compiler/typer.cc
diff --git a/src/compiler/typer.cc b/src/compiler/typer.cc
index f9e65d7da5f3398cac5fb17c77d76af098d606d4..560e3a4cbf7cf20d25910199d25a2c2de5711844 100644
--- a/src/compiler/typer.cc
+++ b/src/compiler/typer.cc
@@ -1681,6 +1681,8 @@ Type* Typer::Visitor::StringFromCodePointTyper(Type* type, Typer* t) {
   return Type::String();
 }
 
+Type* Typer::Visitor::TypeStringCharAt(Node* node) { return Type::String(); }
+
 Type* Typer::Visitor::TypeStringCharCodeAt(Node* node) {
   return typer_->cache_.kUint16;
 }