[X86][Costmodel] Load/store i8 Stride=2 VF=8 interleaving costs

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

Identical to VF=2.

For load we have:
https://godbolt.org/z/4TEbdzbMM - for intels `Block RThroughput: =2.0`; for ryzens, `Block RThroughput: <=1.0`
So pick cost of `2`.

For store we have:
https://godbolt.org/z/MYfzGPf3Y - for intels `Block RThroughput: =1.0`; for ryzens, `Block RThroughput: <=0.5`
So pick cost of `1`.

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/D110705