multiply_sat shift arguments are not immediates

[kpuatamazon]

srai_epi16 expects an immediate shift value, not a variable.
Here's it is called with a variable: https://github.com/kpu/intgemm/blob/61bcbae423eab96156f646a92107ca5300b8ae27/kernels/implementations.inl#L308-L309 And the caller is very much using a variable: https://github.com/kpu/intgemm/blob/61bcbae423eab96156f646a92107ca5300b8ae27/test/kernels/multiply_sat_test.cc#L24-L25

I don't know how newer gcc was fixing it (perhaps srav?) but it's breaking gcc 6.