Changing details as following:
1. The previous order of parameters are irregular and difficult to
   understand. Adjust the order of the parameters according to the
   rule: (RTYPE, input registers, input mask/input index/..., output registers).
   Most of the existing msa macros follow the rule.
2. Remove the redundant macro SLDI_Bn_0 and use SLDI_Bn instead.
Reviewed-by: Shiyou Yin <yinshiyou-hf@loongson.cn>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>