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

[Antonio Frighetto via llvm-commits]

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

If we bail out when `isLegalMaskedGather` returns false, which indeed would happen with such types, – as per @alexey-bataev suggestion above – I don't think we would ever be reaching this point.

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