[turbofan] Introduce a dedicated StringCharAt operator.

src/compiler/simplified-operator.h

Index: src/compiler/simplified-operator.h
diff --git a/src/compiler/simplified-operator.h b/src/compiler/simplified-operator.h
index 4c331136ca408ac9d2a6cf3e38c1a82c957aa85d..e1db1ca5b66a2a144dc0900525fbb621a65d5c0f 100644
--- a/src/compiler/simplified-operator.h
+++ b/src/compiler/simplified-operator.h
@@ -296,6 +296,7 @@ class V8_EXPORT_PRIVATE SimplifiedOperatorBuilder final
   const Operator* StringEqual();
   const Operator* StringLessThan();
   const Operator* StringLessThanOrEqual();
+  const Operator* StringCharAt();
   const Operator* StringCharCodeAt();
   const Operator* StringFromCharCode();
   const Operator* StringFromCodePoint(UnicodeEncoding encoding);