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

[Peter Smith via llvm-commits]

smithp35 wrote:

I think adding that behaviour back makes sense as existing implementations make a defacto standard. Ideally we can trace this back and when we add a test we can say it is for compatibility with GCC.

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