[llvm] [AArch64][TTI] Improve `LegalVF` when computing gather-loads cost (PR #69617)

[David Green via llvm-commits]

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@@ -2989,7 +2989,14 @@ InstructionCost AArch64TTIImpl::getGatherScatterOpCost(
       ElementCount::getScalable(1))
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davemgreen wrote:

It didn't look like that function was called when I tried it, and it is best if these functions work correctly in isolation. It's possible to call it throught the cost model: https://godbolt.org/z/z4635vodq.

I think that because the gather will not become a scalable SVE gather in this case, the line above with useNeonVector should be returning true so we use the BaseT::getGatherScatterOpCost for the cost. 

https://github.com/llvm/llvm-project/pull/69617