[GlobalISel][AArch64] Use getOpcodeDef instead of findMIFromReg
Some minor cleanup.
This function 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.
Differential Revision: https://reviews.llvm.org/D64520