[turbofan] Introduce a dedicated StringCharAt operator.

src/compiler/verifier.cc

Index: src/compiler/verifier.cc
diff --git a/src/compiler/verifier.cc b/src/compiler/verifier.cc
index e89fcc95fbeec2fc1bedbf021fc9ae2ba01ce7b6..fcdfbd9a30f674e48acc18ba3ff89ff5f845aa74 100644
--- a/src/compiler/verifier.cc
+++ b/src/compiler/verifier.cc
@@ -897,6 +897,12 @@ void Verifier::Visitor::Check(Node* node) {
       CheckValueInputIs(node, 1, Type::String());
       CheckTypeIs(node, Type::Boolean());
       break;
+    case IrOpcode::kStringCharAt:
+      // (String, Unsigned32) -> String
+      CheckValueInputIs(node, 0, Type::String());
+      CheckValueInputIs(node, 1, Type::Unsigned32());
+      CheckTypeIs(node, Type::String());
+      break;
     case IrOpcode::kStringCharCodeAt:
       // (String, Unsigned32) -> UnsignedSmall
       CheckValueInputIs(node, 0, Type::String());