replace 2 placeholder uses of undef with poison [NFC]

Author: nunoplopes

llvm/lib/CodeGen/CodeGenPrepare.cpp

@@ -3340,7 +3340,7 @@ class TypePromotionTransaction {
         // Set a dummy one.
         // We could use OperandSetter here, but that would imply an overhead
         // that we are not willing to pay.
-        Inst->setOperand(It, UndefValue::get(Val->getType()));
+        Inst->setOperand(It, PoisonValue::get(Val->getType()));
       }
     }
 

llvm/lib/CodeGen/SjLjEHPrepare.cpp

@@ -267,11 +267,11 @@ void SjLjEHPrepareImpl::lowerIncomingArguments(Function &F) {
 
     Type *Ty = AI.getType();
 
-    // Use 'select i8 true, %arg, undef' to simulate a 'no-op' instruction.
+    // Use 'select i8 true, %arg, poison' to simulate a 'no-op' instruction.
     Value *TrueValue = ConstantInt::getTrue(F.getContext());
-    Value *UndefValue = UndefValue::get(Ty);
+    Value *PoisonValue = PoisonValue::get(Ty);
     Instruction *SI = SelectInst::Create(
-        TrueValue, &AI, UndefValue, AI.getName() + ".tmp", AfterAllocaInsPt);
+        TrueValue, &AI, PoisonValue, AI.getName() + ".tmp", AfterAllocaInsPt);
     AI.replaceAllUsesWith(SI);
 
     // Reset the operand, because it  was clobbered by the RAUW above.