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

[Alfie Richards via llvm-commits]

AlfieRichardsArm wrote:

I have merged the prerequisites. Then as long as the tests pass and @ostannard doesnt have any changes to make I will merge this today. Thank you @s-barannikov @kosarev @statham-arm for your help with this. 

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