Reland of [stubs] Port KeyedLoadIC_Generic stub to TurboFan

src/builtins/builtins-sharedarraybuffer.cc

Index: src/builtins/builtins-sharedarraybuffer.cc
diff --git a/src/builtins/builtins-sharedarraybuffer.cc b/src/builtins/builtins-sharedarraybuffer.cc
index 23d4f43af29f7c8de96fced22e2d39dce0ff8dee..cd9dc07e0c6eb8ca2561ab351123ccd959b91712 100644
--- a/src/builtins/builtins-sharedarraybuffer.cc
+++ b/src/builtins/builtins-sharedarraybuffer.cc
@@ -141,6 +141,7 @@ void ValidateAtomicIndex(CodeStubAssembler* a, compiler::Node* index_word,
   using namespace compiler;
   // Check if the index is in bounds. If not, throw RangeError.
   CodeStubAssembler::Label if_inbounds(a), if_notinbounds(a);
+  // TODO(jkummerow): Use unsigned comparison instead of "i<0 || i>length".
   a->Branch(
       a->WordOr(a->Int32LessThan(index_word, a->Int32Constant(0)),
                 a->Int32GreaterThanOrEqual(index_word, array_length_word)),