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

[Alfie Richards via llvm-commits]

AlfieRichardsArm wrote:

@s-barannikov @kosarev With the latest fix to https://github.com/llvm/llvm-project/pull/83587 this should now pass and be ready to merge

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