Add NOP to ARM IR lowering.

src/IceTargetLoweringARM32.cpp

Index: src/IceTargetLoweringARM32.cpp
diff --git a/src/IceTargetLoweringARM32.cpp b/src/IceTargetLoweringARM32.cpp
index 0322a7e54ef60c6a77ff3599f78f52998d4a1620..8f18c9e250d0cfe16099c6f3dceb0f1a5479f77b 100644
--- a/src/IceTargetLoweringARM32.cpp
+++ b/src/IceTargetLoweringARM32.cpp
@@ -5313,7 +5313,7 @@ void TargetARM32::randomlyInsertNop(float Probability,
                                     RandomNumberGenerator &RNG) {
   RandomNumberGeneratorWrapper RNGW(RNG);
   if (RNGW.getTrueWithProbability(Probability)) {
-    UnimplementedError(Func->getContext()->getFlags());
+    _nop();
   }
 }