[llvm] [VectorCombine] Fold Deinterleave/Interleave Pairs (PR #211022)

[Benjamin Maxwell via llvm-commits]

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MacDue wrote:

Possibly, this should also do `propagateMetadata(NewValue, Step.Insts)`. Though the only case I think we could hit is the `fpmath` metadata (though I've not checked). 

https://github.com/llvm/llvm-project/pull/211022