R600/SI: Use V_ADD_F32 instead of V_MOV_B32 for clamp/neg/abs modifiers.

The modifiers don't seem to have any effect with V_MOV_B32, supposedly it's
meant to just move bits untouched.

Fixes 46 piglit tests with radeonsi, though unfortunately 11 of those had
just regressed because they started using the clamp modifier.

NOTE: This is a candidate for the Mesa stable branch.

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>