[llvm] [AArch64][llvm] Add missing form for `LD64B`/`ST64B` instructions (PR #196301)

[Oliver Stannard via llvm-commits]

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

@CarolineConcatto: It's possible that D91775 was based on an early version of the spec which didn't have the optional zero offset syntax, or it could have just been an accidental omission. In either case it's in the current reference manual, so this patch LGTM.

https://github.com/llvm/llvm-project/pull/196301