[llvm] [RFC] TableGen-erate SDNode descriptions (PR #119709)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

I've attempted to continue the AArch64 patch in #140472 (in line with https://discourse.llvm.org/t/rfc-tablegen-erating-sdnode-descriptions/83627/8 :slightly_smiling_face:) 

https://github.com/llvm/llvm-project/pull/119709