[X86] `X86TTIImpl::getInterleavedMemoryOpCostAVX512()`: mask is i8 not i1

Even though AVX512's masked mem ops (unlike AVX1/2) have a mask
that is a `VF x i1`, replication of said masks happens after
promotion of it to `VF x i8`, so we should use `i8`, not `i1`,
when calculating the cost of mask replication.