X87: [Atomics] Remove Atomics code stubs; use TF ops.

src/compiler/x87/code-generator-x87.cc

Index: src/compiler/x87/code-generator-x87.cc
diff --git a/src/compiler/x87/code-generator-x87.cc b/src/compiler/x87/code-generator-x87.cc
index ef3e3631f9dd2376d7dc3b794f8dc955c7f1ee8b..829f13ee061c65dd2d756aac23fdb241bcafe8cf 100644
--- a/src/compiler/x87/code-generator-x87.cc
+++ b/src/compiler/x87/code-generator-x87.cc
@@ -1775,6 +1775,13 @@ 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)