[X86] getShuffleCost - recognise concat_vector(X,Y) shuffle as InsertSubvector...

[X86] getShuffleCost - recognise concat_vector(X,Y) shuffle as InsertSubvector instead of PermuteTwoSrc

We don't have a concat_vector shuffle kind and improveShuffleKindFromMask won't alter the base type to match it as InsertSubvector.

But since this is how X86 will lower concat_vector anyhow, just recognise it explicitly.

Another step for #67803