from xtensa.c:55
#define _XT_INS_FORMAT_RRR(X, OPCODE, ST, R) \
(XT_ISBE(X) ? (XT_NIBSWAP24(OPCODE) \
| ((XT_NIBSWAP8((ST) & 0xFF)) << 12) \
| (((R) & 0x0F) << 8)) << 8 \
: (OPCODE) \
| (((ST) & 0xFF) << 4) \
| (((R) & 0x0F) << 12))