[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

llvm-svn: 365684