Revert of [Atomics] Remove Atomics code stubs; use TF ops

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

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/code-generator.h"
 
 #include "src/ast/scopes.h"
 #include "src/compiler/code-generator-impl.h"
 #include "src/compiler/gap-resolver.h"
 #include "src/compiler/node-matchers.h"
@@ skipping 1749 matching lines @@
       break;
     case kCheckedStoreFloat32:
       ASSEMBLE_CHECKED_STORE_FLOAT(Movss);
       break;
     case kCheckedStoreFloat64:
       ASSEMBLE_CHECKED_STORE_FLOAT(Movsd);
       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)
 
 
 // Assembles branches after this instruction.
 void CodeGenerator::AssembleArchBranch(Instruction* instr, BranchInfo* branch) {
   X64OperandConverter i(this, instr);
   Label::Distance flabel_distance =
       branch->fallthru ? Label::kNear : Label::kFar;
   Label* tlabel = branch->true_label;
@@ skipping 492 matching lines @@
     int padding_size = last_lazy_deopt_pc_ + space_needed - current_pc;
     __ Nop(padding_size);
   }
 }
 
 #undef __
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8