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

[Alfie Richards via llvm-commits]

AlfieRichardsArm wrote:

@mstorsjo I have a fix I will add you to shortly. Thank you for finding this. 
Do you have the compile command for the whole file? I would like to check that the file now compiles.

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