[NFC][MC][ARM] Reorder decoder functions N/N (#158767)
Move `DecodeT2AddrModeImm8` and `DecodeT2Imm8` definition before its
first use and eliminate the last remaining forward declarations of
decode functions.
Work on https://github.com/llvm/llvm-project/issues/156560 : Reorder ARM
disassembler decode functions to eliminate forward declarations
