[llvm] [NFC][MC][XCore] Rearrange decoder functions for XCore disassembler (PR #155009)

[Nigel Perks via llvm-commits]

nigelp-xmos wrote:

Happy to approve, but if this combines general tidying up of forward declarations, with changes enabled by `fieldFromInstruction`, wouldn't it be clearer in separate commits, or is there a reason to combine them? Minimizing NFCs?

https://github.com/llvm/llvm-project/pull/155009