[X86][Costmodel] Load/store i16 Stride=6 VF=8 interleaving costs

The only sched models that for cpu's that support avx2
but not avx512 are: haswell, broadwell, skylake, zen1-3

For load we have:
https://godbolt.org/z/3Tc5s897j - for intels `Block RThroughput: =39.0`; for ryzens, `Block RThroughput: <=13.5`
So pick cost of `39`.

For store we have:
https://godbolt.org/z/fo1h9E67e - for intels `Block RThroughput: =21.0`; for ryzens, `Block RThroughput: <=12.0`
So pick cost of `21`.

I'm directly using the shuffling asm the llc produced,
without any manual fixups that may be needed
to ensure sequential execution.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D110592