[llvm] [TableGen] List the indices of sub-operands (PR #163723)

[Simon Tatham via llvm-commits]

statham-arm wrote:

Background: in a PR of mine last week (#163217) @sparker-arm suggested that it would be nice if I hadn't had to write `index + 3` for one of these sub-operand offsets. That PR is abandoned while I work on solving the same problem another way, but I'm still going to need to do that part of the job, and it would be nice if there were a constant available for the job next time :slightly_smiling_face: 

https://github.com/llvm/llvm-project/pull/163723