[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 okay, very strange. I cant find any references to this in specifications or docs. Do you have any documentation for it? 

I would be happy to add some aliases for this alternate form if it's correct behavior, but I can't seem to find documentation for it or references to it before this code.

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