[X86][Costmodel] Load/store i8 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/v98qPTTf6 - for intels `Block RThroughput: =18.0`; for ryzens, `Block RThroughput: =6.0`
So pick cost of `18`.

For store we have:
https://godbolt.org/z/rn5T9E8q6 - for intels `Block RThroughput: <=16.0`; for ryzens, `Block RThroughput: <=4.5`
So pick cost of `16`.

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/D111011