Revert of [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 299af40b6c3ec80684f82a775e68ea925efa471e..c471465101fe12fc4ecd86b40a818ddcf32df958 100644
--- a/src/compiler/ia32/code-generator-ia32.cc
+++ b/src/compiler/ia32/code-generator-ia32.cc
@@ -1324,13 +1324,6 @@
     case kCheckedLoadWord64:
     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)