[llvm] [SLP] Extend compare chains downward before bottom-up vectorization (PR #210921)

[Alexey Bataev via llvm-commits]

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+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
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alexey-bataev wrote:

The new tests should be precommitted in separate NFC patch

https://github.com/llvm/llvm-project/pull/210921