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

[Alfie Richards via llvm-commits]

AlfieRichardsArm wrote:

Looks like the test failures were coming from ambiguous matchables being sorted differently. I have made a new patch that disambiguates these cases here: https://github.com/llvm/llvm-project/pull/84483

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