Moves the small non-public parts of core/ir/*/opcode.h into instr.h and opnd.h.
Cleans up core/ir/*/opcode.h and codec.py's generated content such that these files are now just directly copied to the public include/ directory.
Renames opcode.h to opcode_api.h for all 3 arches to make it clear they are part of the public interface.
Issue: #3092 (closed)