Revert of [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 7f2a6c23abe9d38852f25d10232cb255fabae95a..e756ee2b2b99a46ba6ea45aec1066d06835a0eab 100644
--- a/src/compiler/x64/code-generator-x64.cc
+++ b/src/compiler/x64/code-generator-x64.cc
@@ -1766,13 +1766,6 @@
       break;
     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)