Add lfence after ret in retpoline thunk.

Index: lib/Target/X86/X86IndirectThunks.cpp
--- lib/Target/X86/X86IndirectThunks.cpp.orig
+++ lib/Target/X86/X86IndirectThunks.cpp
@@ -249,6 +249,7 @@ void RetpolineThunkInserter::populateThunk(MachineFunc
 
   CallTarget->back().setPreInstrSymbol(MF, TargetSym);
   BuildMI(CallTarget, DebugLoc(), TII->get(RetOpc));
+  BuildMI(CallTarget, DebugLoc(), TII->get(X86::LFENCE));
 }
 
 FunctionPass *llvm::createX86IndirectThunksPass() {