[AMDGPU] Try decoding instructions longest first. NFCI. (#82014)

AMDGPUDisassembler::getInstruction tries decoding instructions using
different DecoderTables in a confusing order: first 96-bit instructions,
then some 64-bit, then 32-bit, then some more 64-bit.

This patch changes it to always try longer encodings first. The
motivation is to make getInstruction easier to understand, and to pave
the way for combining some 64-bit tables that do not need to be
separate.