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

[Phoebe Wang via llvm-commits]

phoebewang wrote:

Does this conflict with https://github.com/llvm/llvm-project/pull/199488? Another approch would be adding `TuningPreferNoGather` like https://github.com/llvm/llvm-project/pull/205992 does. It has minor side effect to existing tests.

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