[turbofan] Introduce a dedicated StringCharAt operator.

src/compiler/verifier.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/verifier.h"
 
 #include <algorithm>
 #include <deque>
 #include <queue>
 #include <sstream>
@@ skipping 879 matching lines @@
       CheckTypeIs(node, Type::Number());
       break;
     case IrOpcode::kStringEqual:
     case IrOpcode::kStringLessThan:
     case IrOpcode::kStringLessThanOrEqual:
       // (String, String) -> Boolean
       CheckValueInputIs(node, 0, Type::String());
       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());
       CheckValueInputIs(node, 1, Type::Unsigned32());
       CheckTypeIs(node, Type::UnsignedSmall());
       break;
     case IrOpcode::kStringFromCharCode:
       // Number -> String
       CheckValueInputIs(node, 0, Type::Number());
       CheckTypeIs(node, Type::String());
@@ skipping 760 matching lines @@
          replacement->op()->EffectOutputCount() > 0);
   DCHECK(!NodeProperties::IsFrameStateEdge(edge) ||
          replacement->opcode() == IrOpcode::kFrameState);
 }
 
 #endif  // DEBUG
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8