Why not? The rules for validating Bitcoin transactions and blocks must be completely specified and followed exactly by all implementations of the client software, otherwise you risk forks.
Bitcoin is a consensus system. It is paramount that all participating nodes reach the exact same state.
If undefined behavior could make some nodes accept a transaction while others reject it the state would become inconsistent and potentially mutually exclusive. From one currency, you'd have two and every coin could be spent twice.
There is nothing that indicates undefined behavior of LLVM would lead to a fork in the blockchain.