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

[Alfie Richards via llvm-commits]

AlfieRichardsArm wrote:

@smithp35 Ah okay, in that case shall I re-add this behavior and add some tests? 
I dislike the way `strexd` and `ldrexd` is handled so could rewrite this to handle this case and have better diagnostics.

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