[IR] Use poison instead of undef for self-referential phi

Author: nikic

llvm/lib/IR/Instructions.cpp

@@ -205,7 +205,7 @@ Value *PHINode::hasConstantValue() const {
       ConstantValue = getIncomingValue(i);
     }
   if (ConstantValue == this)
-    return UndefValue::get(getType());
+    return PoisonValue::get(getType());
   return ConstantValue;
 }