[X86][Costmodel] `getReplicationShuffleCost()`: promote 16 bit-wide elements to 32...

[X86][Costmodel] `getReplicationShuffleCost()`: promote 16 bit-wide elements to 32 bit when no AVX512BW

The basic idea is simple, if we don't have native shuffle for this element type,
then we must have native shuffle for wider element type,
so promote, replicate, demote.

I believe, asking `getCastInstrCost(Instruction::Trunc` is correct semantically,
case in point `trunc <32 x i32> to <32 x i8>` aka 2 * ZMM will naively result in
2 * XMM, that then will be packed into 1 * YMM,
and it should count the cost of said packing,
not just the truncations.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D113609