target-mips: split decode_opc_special* into *_r6 and *_legacy

target-mips: split decode_opc_special* into *_r6 and *_legacy

For better code readability and to avoid 'if' statements for all R6 and preR6
instructions whose opcodes are the same - decode_opc_special* functions are
split into functions with _r6 and _legacy suffixes.

*_r6 functions will contain instructions which were introduced in R6.
*_legacy functions will contain instructions which were removed in R6.

Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>