[llvm] [SelectionDAG] Freeze dynamic vector indices lowered through the stack (PR #225031)

[Mahmoud Naderi via llvm-commits]

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slant14 wrote:

Yes, that's the better place. I'll move the freeze into `getVectorSubVecPointer` and use the `UndefPoisonKind::PoisonOnly` overload so constant indices keep the fast path in `clampDynamicVectorIndex`.

https://github.com/llvm/llvm-project/pull/225031