[llvm] [AArch64] TableGen-erate SDNode descriptions (PR #140472)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> Can this PR be broken up? Removing unused nodes/code can be done in isolation? The same is hopefully true for things like the REV fix and the new documentation added to the td file.

Sure, I'll split off some of the fixes/NFCs soon :+1: 

https://github.com/llvm/llvm-project/pull/140472