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

[Sourabh Singh Tomar via llvm-commits]

SouraVX wrote:

> Does this conflict with #199488? Another approch would be adding `TuningPreferNoGather` like #205992 does. It has minor side effect to existing tests.

But that still leaves avx512 cost path ambiguous with the assumption that every avx512 has `fastgather` baked in.

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