[llvm] [AArch64] Remove post-decoding instruction mutations (PR #156364)

[David Green via llvm-commits]

https://github.com/davemgreen approved this pull request.

I'm not the biggest expert here, and bits<0> seems a little unusual, but as far as I can tell this LGTM. Thanks

https://github.com/llvm/llvm-project/pull/156364