[Atomics] Remove Atomics code stubs; use TF ops

src/compiler/ia32/code-generator-ia32.cc

Index: src/compiler/ia32/code-generator-ia32.cc
diff --git a/src/compiler/ia32/code-generator-ia32.cc b/src/compiler/ia32/code-generator-ia32.cc
index c471465101fe12fc4ecd86b40a818ddcf32df958..299af40b6c3ec80684f82a775e68ea925efa471e 100644
--- a/src/compiler/ia32/code-generator-ia32.cc
+++ b/src/compiler/ia32/code-generator-ia32.cc
@@ -1325,6 +1325,13 @@ void CodeGenerator::AssembleArchInstruction(Instruction* instr) {
     case kCheckedStoreWord64:
       UNREACHABLE();  // currently unsupported checked int64 load/store.
       break;
+    case kAtomicLoadInt8:
+    case kAtomicLoadUint8:
+    case kAtomicLoadInt16:
+    case kAtomicLoadUint16:
+    case kAtomicLoadWord32:
+      UNREACHABLE();  // Won't be generated by instruction selector.
+      break;
   }
 }  // NOLINT(readability/fn_size)