vp8,vpx_dsp:[loongson] fix specification of instruction name

vp8,vpx_dsp:[loongson] fix specification of instruction name

1.'xor,or,and' to 'pxor,por,pand'. In the case of operating FPR,
  gcc supports both of them, clang only supports the second type.
2.'dsrl,srl' to 'ssrld,ssrlw'. In the case of operating FPR, gcc
  supports both of them, clang only supports the second type.

Change-Id: I93b47348e7c6580d99f57dc11165b4645236533c