Merge changes I7d68c7f2,If283fc08,I3b1e0a2c into main

* changes:
  vp9[loongarch]: Optimize avg_variance64x64/variance8x8
  vp9[loongarch]: Optimize fdct4x4/8x8_lsx
  vp9[loongarch]: Optimize vpx_hadamard_16x16/8x8