[compiler] Make StringRef::GetFirstChar() return an optional value

Follow-up that continues to make StringRefs's methods return optional
values.

Bug: v8:7790
Change-Id: I34f907810747216b047a3da8db035cf4f62728c3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2615162Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71982}

src/compiler/heap-refs.h

@@ -835,7 +835,7 @@ class StringRef : public NameRef {
   Handle<String> object() const;
 
   base::Optional<int> length() const;
-  uint16_t GetFirstChar();
+  base::Optional<uint16_t> GetFirstChar();
   base::Optional<double> ToNumber();
   bool IsSeqString() const;
   bool IsExternalString() const;

src/compiler/js-call-reducer.cc

@@ -6040,7 +6040,7 @@ Reduction JSCallReducer::ReduceStringPrototypeStartsWith(Node* node) {
               graph()->NewNode(simplified()->StringCharCodeAt(), receiver,
                                masked_position, etrue, if_true);
 
-          Node* search_first = jsgraph()->Constant(str.GetFirstChar());
+          Node* search_first = jsgraph()->Constant(str.GetFirstChar().value());
           vtrue = graph()->NewNode(simplified()->NumberEqual(), string_first,
                                    search_first);
         }

src/compiler/js-heap-broker.cc

@@ -3079,8 +3079,16 @@ base::Optional<int> StringRef::length() const {
   return data()->AsString()->length();
 }
 
-uint16_t StringRef::GetFirstChar() {
+base::Optional<uint16_t> StringRef::GetFirstChar() {
   if (data_->should_access_heap()) {
+    if (data_->kind() == kNeverSerializedHeapObject &&
+        !this->IsInternalizedString()) {
+      TRACE_BROKER_MISSING(
+          broker(),
+          "first char for kNeverSerialized non-internalized string " << *this);
+      return base::nullopt;
+    }
+
     if (broker()->local_isolate()) {
       return object()->Get(0, broker()->local_isolate());
     } else {

src/compiler/typed-optimization.cc

@@ -482,7 +482,8 @@ TypedOptimization::TryReduceStringComparisonOfStringFromSingleCharCode(
         simplified()->NumberBitwiseAnd(), from_char_code_repl,
         jsgraph()->Constant(std::numeric_limits<uint16_t>::max()));
   }
-  Node* constant_repl = jsgraph()->Constant(string.GetFirstChar());
+  if (!string.GetFirstChar().has_value()) return NoChange();
+  Node* constant_repl = jsgraph()->Constant(string.GetFirstChar().value());
 
   Node* number_comparison = nullptr;
   if (inverted) {