Draft: r300: run merge_channels pass search also backwards

To account for scenarios where we can't merge the first instruction down but we can do the opposite.

Shader-db:

RV370:
total instructions in shared programs: 83853 -> 83519 (-0.40%)
instructions in affected programs: 20215 -> 19881 (-1.65%)
helped: 238
HURT: 0

RV530:
total instructions in shared programs: 131800 -> 131410 (-0.30%)
instructions in affected programs: 22360 -> 21970 (-1.74%)
helped: 276
HURT: 0

BTW @gawin I'm taking the #7693 (comment 1642870) comment as a tested-by :-)