[llvm] [SLP] Check if instructions exist after vectorization (PR #120434)

[via llvm-commits]

llvmbot wrote:

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Failed to cherry-pick: e7a4d78ad328d02bf515b2fa4af8b2c188a6a636

https://github.com/llvm/llvm-project/actions/runs/12402342030

Please manually backport the fix and push it to your github fork.  Once this is done, please create a [pull request](https://github.com/llvm/llvm-project/compare)

https://github.com/llvm/llvm-project/pull/120434