target/openrisc: Invert the decoding in dec_calc

Decoding the opcodes in the right order reduces by 100+ lines.
Also, it happens to put the opcodes in the same order as Chapter 17.

Reviewed-by: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Signed-off-by: Richard Henderson <rth@twiddle.net>