[X86][Costmodel] Load/store i8 Stride=6 VF=2 interleaving costs

[X86][Costmodel] Load/store i8 Stride=6 VF=2 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/jvj6jzns5 - for intels `Block RThroughput: =6.0`; for ryzens, `Block RThroughput: <=3.0`
So pick cost of `6`.

For store we have:
https://godbolt.org/z/ros7eebMP - for intels `Block RThroughput: =7.0`; for ryzens, `Block RThroughput: <=3.0`
So pick cost of `7`.

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/D111008