lavu/opencl:optimize compile kernel operation

Reviewed-by: Stefano Sabatini <stefasab@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
joinfields filter

Signed-off-by: Paul B Mahol <onemda@gmail.com>
astats filter

Signed-off-by: Paul B Mahol <onemda@gmail.com>