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

[Alfie Richards via llvm-commits]

AlfieRichardsArm wrote:

Ah, looking at file that caused the failure (llvm/test/MC/ARM/negative-immediates-fail.s) we only previously checked for 1 of the errors. So if they were in different orders it wouldn't have caused a failure. So this reordering was probably happening previously but wasn't tested for.

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