Some minor cleanup. Trying to make use of functions that live in Utils more.
getOpcodeDef in Utils does the same thing as findMIFromReg. It also looks through copies, which findMIFromReg didn't.
Delete findMIFromReg and use getOpcodeDef instead. This only happens in tryOptVectorDup right now.
Update opt-shuffle-splat to show that we can look through the copies now, too.
This doesn't really impact code size anywhere in real life as far as I've seen, but it's nice to improve code reuse.