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:

intgemm/kernels/implementations.inl

Lines 308 to 309 in 61bcbae

	auto low = srai_epi16(multiply<int16_t>(upcasted_a.first, upcasted_b.first), right_shift);
	auto hi = srai_epi16(multiply<int16_t>(upcasted_a.second, upcasted_b.second), right_shift);

And the caller is very much using a variable:

intgemm/test/kernels/multiply_sat_test.cc

Lines 24 to 25 in 61bcbae

	for (std::size_t shift = 0; shift <= 2 * 8 * sizeof(Type_); ++shift) {
	*output.template as<vec_t>() = kernels::multiply_sat<Type_>(*input1.template as<vec_t>(), *input2.template as<vec_t>(), shift);

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