[llvm] Change the ARM mnemonic operands to be optional for ASM parsing (PR #83436)

[Alfie Richards via llvm-commits]

AlfieRichardsArm wrote:

This change is intended to have minor/no user facing changes. It allows for the removal of some hacks and makes the parsing of ARM more idiomatic with the way TableGen works.

Additionally, it is part of a larger piece of work to address the numerous flaws with how we handle ".w" and ".n".
For more context see: 
https://github.com/llvm/llvm-project/issues/14175
https://gist.github.com/rprichard/2a601c3dd1b281f953b4e08b5a9361bb

https://github.com/llvm/llvm-project/pull/83436