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

[Alfie Richards via llvm-commits]

AlfieRichardsArm wrote:

Hmmm, the same problem remains irritatingly. There are cases of matchables with the same operands, features, but _different_ result instructions, which cause different diagnostics....... 
So I'm adding a sort for that also. To aforementioned commit.

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