[llvm] [X86][CostModel] Decouple AVX512 from fast-gather in getGatherOverhead() (PR #206506)

[Phoebe Wang via llvm-commits]

phoebewang wrote:

I'd like the [planned implementation](https://github.com/llvm/llvm-project/pull/199488#issuecomment-4658264239) the most. And I don't see anything more the patch can provide beyond. Even the current #199488 is more complete that this one. Besides, this has to add a lot of changes to existing tests, which is unnecessary.

https://github.com/llvm/llvm-project/pull/206506