[X86][Costmodel] Load/store i8 Stride=2 VF=16 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/a9hv4z47v - for intels `Block RThroughput: =4.0`; for ryzens, `Block RThroughput: =2.0`
So pick cost of `4`.

For store we have:
https://godbolt.org/z/6GfPn1b79 - for intels `Block RThroughput: =3.0`; for ryzens, `Block RThroughput: <=2.0`
So pick cost of `3`.

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/D110708