loongarch: Reduce the number of instructions

loongarch: Reduce the number of instructions

Replace some redundant instructions to improve the efficiency
of the program.

1. txfm_macros_lsx.h
2. vpx_convolve8_avg_lsx.c
3. vpx_convolve8_horiz_lsx.c
4. vpx_convolve8_lsx.c
5. vpx_convolve8_vert_lsx.c
6. vpx_convolve_copy_lsx.c
7. vpx_convolve_lsx.h

Bug: webm:1755

Change-Id: I9b7fdf6900338a26f9b1775609ad387648684f3d