[Atomics] Remove Atomics code stubs; use TF ops

src/compiler/x64/code-generator-x64.cc

Index: src/compiler/x64/code-generator-x64.cc
diff --git a/src/compiler/x64/code-generator-x64.cc b/src/compiler/x64/code-generator-x64.cc
index e756ee2b2b99a46ba6ea45aec1066d06835a0eab..7f2a6c23abe9d38852f25d10232cb255fabae95a 100644
--- a/src/compiler/x64/code-generator-x64.cc
+++ b/src/compiler/x64/code-generator-x64.cc
@@ -1767,6 +1767,13 @@ void CodeGenerator::AssembleArchInstruction(Instruction* instr) {
     case kX64StackCheck:
       __ CompareRoot(rsp, Heap::kStackLimitRootIndex);
       break;
+    case kAtomicLoadInt8:
+    case kAtomicLoadUint8:
+    case kAtomicLoadInt16:
+    case kAtomicLoadUint16:
+    case kAtomicLoadWord32:
+      UNREACHABLE();  // Won't be generated by instruction selector.
+      break;
   }
 }  // NOLINT(readability/fn_size)