NFC: use AtomicOrdering isStrongerThan

[jfb]

Summary

As discussed in D18775.

lib/CodeGen/CGAtomic.cpp

if (llvm::ConstantInt *FO = dyn_cast<llvm::ConstantInt>(FailureOrderVal)) {
     case AtomicExpr::AO_ABI_memory_order_consume:
     case AtomicExpr::AO_ABI_memory_order_acquire:
       FailureOrder = llvm::AtomicOrdering::Acquire;
       break;
     case AtomicExpr::AO_ABI_memory_order_seq_cst:
       FailureOrder = llvm::AtomicOrdering::SequentiallyConsistent;
       break;
     }
-    if (FailureOrder >= SuccessOrder) {
-      // Don't assert on undefined behaviour.
+    if (isStrongerThan(FailureOrder, SuccessOrder)) {
+      // Don't assert on undefined behavior "failure argument shall be no
+      // stronger than the success argument".
       FailureOrder =
         llvm::AtomicCmpXchgInst::getStrongestFailureOrdering(SuccessOrder);
     }
     emitAtomicCmpXchg(CGF, E, IsWeak, Dest, Ptr, Val1, Val2, Size,
                       SuccessOrder, FailureOrder);
     return;
   }
 
auto SuccessFailureRVal = emitAtomicLibcall(CGF, "__atomic_compare_exchange",
                                               CGF.getContext().BoolTy, Args);
 
   return SuccessFailureRVal.getScalarVal();
 }
 
 std::pair<RValue, llvm::Value *> AtomicInfo::EmitAtomicCompareExchange(
     RValue Expected, RValue Desired, llvm::AtomicOrdering Success,
     llvm::AtomicOrdering Failure, bool IsWeak) {
-  if (Failure >= Success)
-    // Don't assert on undefined behavior.
+  if (isStrongerThan(Failure, Success))
+    // Don't assert on undefined behavior "failure argument shall be no stronger
+    // than the success argument".
     Failure = llvm::AtomicCmpXchgInst::getStrongestFailureOrdering(Success);
 
   // Check whether we should use a library call.
   if (shouldUseLibcall()) {
     // Produce a source address.
     Address ExpectedAddr = materializeRValue(Expected);
     Address DesiredAddr = materializeRValue(Desired);
     auto *Res = EmitAtomicCompareExchangeLibcall(ExpectedAddr.getPointer(),