[llvm] [NFC][ARM][MC] Rearrange decoder functions 2/N (PR #155464)

[Rahul Joshi via llvm-commits]

jurahul wrote:

not, I am attempting to do this in steps where the in the diff is clearly visible that each delta is a simple rearranging of existing code. Maybe I can stack more commits here so that each commit individually has that property? I can confirm for example that the next diff after this to move ARMDisassembler::AddThumbPredicate shows a clean diff

https://github.com/llvm/llvm-project/pull/155464