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

[Alfie Richards via llvm-commits]

AlfieRichardsArm wrote:

@statham-arm if you have time could you give it a brief look over. Its changed quite a bit since you last saw it.

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