[turbofan] Speculatively optimize string character access.

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index b9f633a43990dc6fef572677f5cdca8a7bb718c8..8da8036ea570b9345a68399015a6971a815d2418 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -2855,6 +2855,10 @@ void Heap::CreateInitialObjects() {
       handle(Smi::FromInt(Isolate::kProtectorValid), isolate()));
   set_species_protector(*species_cell);
 
+  Handle<Cell> string_bounds_check_cell = factory->NewCell(
+      handle(Smi::FromInt(Isolate::kProtectorValid), isolate()));
+  set_string_bounds_check_protector(*string_bounds_check_cell);
+
   cell = factory->NewPropertyCell();
   cell->set_value(Smi::FromInt(Isolate::kProtectorValid));
   set_string_length_protector(*cell);